The group of protozoa is the evolutionary ancestor of multicellular organisms. Kingdom of Animals

How did the transition from unicellular to multicellular occur during the development of the animal world? This question cannot be considered to some extent solved, and one has to confine oneself to more or less probable hypotheses.

1 - a colony of collar flagellates of the Sphaeroeca type with monotomic reproduction, 2 - a colony of collar flagellates of the Proterospongia type with palintomic reproduction and sexual process, 3 - early phagocytella I without a mouth, 4 - lamellar (Placozoa) without a mouth, 5 - without a sponge (Spongia) and intestines, 6 - late phagocytella II with the mouth, 7 - primary coelenterates type of gastrea (bilayer with the mouth), 8 - primary turbellaria (type
Plathelminthes) - parenchymal with the mouth shifted to the abdominal side, three-layered, 9 - intestinal turbellaria with further differentiation of cells and displacement of the mouth to the abdominal side

The oldest and most widespread among zoologists hypothesis is that colonial organisms similar to colonial flagella were transitional to multicellular forms. Among these organisms there are those that consist of several completely similar cells, without traces of any cellular differentiation (Gonium, Pandorina, etc.). Such an organism can be considered as a colony of separated, but not dispersed cells. In this case, it is assumed that at first the colonies consisted of identical cells, and then the differentiation of cellular elements arose.

In the 70s of the last century, E. Haeckel, using the data of embryology, and especially the work of the Russian zoologist A.O. Kovalevsky, developed a theory of the origin of multicellular organisms, called the theory of gastrea.

E. Haeckel - the author of the biogenetic law (formulated by him almost simultaneously with F. Müller), according to which "ontogeny is a brief repetition of phylogeny" - saw in all stages of egg cleavage a repetition of the features of the disappeared ancestors of multicellular animals. The first hypothetical unicellular (amoeboid) ancestor corresponding to the egg stage was named Cytea. From him, according to Haeckel, all naked organisms originated. A spherical colony of amoeboid cells (organisms) turned into a single organism - the sea, which corresponded to the morula stage. The next hypothetical ancestor - blastea - arose as a result of the accumulation of gelatinous substance in the center (colony) of the seas and the distribution of its cells (members of the colony) along the periphery. In embryonic development, it corresponds to the blastula stage. The hypothetical blastea initially moved with the help of pseudopodia, which later turned into flagella. Finally, gastria arose by invagination of the anterior wall of the blastea. Outside, gastric cells continued to carry flagella, which ensure its movement. The inner layer of cells has lost the flagella and turned into the primary intestine. The site of invagination was given by the primary mouth, with the help of which the intestinal - gastric - cavity communicated with the external environment. The digestion of food took place in the gastric cavity.

The outer layer of gastrea gave its descendants ectoderm, the inner layer - endoderm. Thus, according to Haeckel's theory, all multicellular animals, including sponges, descended from one progenitor form - gastrea. They inherited from her two primary germ layers - ento- and ectoderm - and the primary intestine. All tissues and organs of multicellular organisms later developed from these formations. The skin and intestines are homologous in all multicellular organisms, since they have a common origin. Haeckel's theory won numerous adherents and dominated science for a long time, but at the same time provoked fair criticism.

The origin of multicellular organisms according to I.I.Mechnikov

I. I. Mechnikov was one of the serious opponents of this theory. His most significant objections to Haeckel were the following: 1. The formation of gastrula by intussusception cannot be considered primary, since in the most primitive multicellular organisms (coelenterates, nonintestinal turbellaria), gastrulation occurs by multiple immigration of cells into the blastula cavity. 2. The formation of the primary intestine with cavity digestion could not be primary, since intracellular digestion is to a large extent characteristic of the lower multicellular ones. 3. The process of invagination in phylogeny could not be due to either physiological or environmental reasons... II Mechnikov assumed that the ancestor of multicellular animals (Metazoa) was a colony of flagellates. The primary multicellular organism was single-layered and spherical (blastea, according to Haeckel), covered with flagella. The same cells performed the functions of movement and absorption of food. After the capture of food particles, the cells lost their flagella and left the surface to the interior of the body. There, food was digested, after which the cells could return to the surface and form a new flagellum. Thus, there was a primary, facultative (temporary) isolation of the outer layer of cells - kinoblast, which have the function of movement, and the internal mass of cells - phagocytoblast, engaged in digestion. As a result of evolution, this division was fixed and the ancestor of all multicellular organisms was formed - parenchymella, or phagocytella (the second name was used by II Mechnikov later).

Phagocytella reproduced sexually. Fertilized eggs underwent complete uniform crushing. The descendants of the phagocytella, when settling to the bottom and transitioning to an attached way of life, gave rise to a branch going to the sponges. Floating phagocytellae later developed into primary coelenterates, and from the phagocytoblast they formed a primary intestine with a mouth opening. Some of the descendants of the phagocytella passed on to life at the bottom; in crawling forms, the body was flattened, bilateral symmetry arose, from which primary intestinal ciliary worms arose.

The hypothesis of I.I. He was the first to pose the important problem of the evolution of ontogenesis itself, changes in the methods of gastrulation and cell differentiation in different groups of lower coelenterates. He introduced a lot of new things into the doctrine of the primordial germ layers and their evolution.

Relatively recently, another hypothesis for the origin of multicellularity has been put forward, called polyenergy or the hypothesis of cellulization. Its author is the scientist I. Khadzhi. At first, he considered the ancestors of multicellular animals to be multinucleated flagellates, with a large number of flagella, and later to ciliate-like forms (ciliates, before the emergence of nuclear dualism in them). From them, according to Khadzhi, two branches of the animal world went - one to modern ciliates, the other to the most primitive (in his opinion) multicellular - intestinal ciliary worms (Acoela). I. Khadzhi compared the structure of ciliates and intestinal turbellaria and found many external similarities between them. From this, he concludes that the organelles of protozoa have turned into organs of multicellular organisms, while an increase (multiplication) in the number of nuclei and the subsequent isolation of plasma around them (cellulization) led to the emergence of multicellularity. In modern Acoela, according to the author, this process has not yet ended, which is why the endoderm of these animals has a state of plasmodium. In reality, this state of syncytium, the absence of boundaries between cells, arises in these animals a second time, in the process of ontogenesis, and not in all species. Recently, it has been possible to confirm the true cellular structure of Acoela; using an electron microscope were found cell membranes in their outer epithelium.

A comparative analysis of the body structure of ciliates and non-intestinal turbellaria showed that it is impossible to carry out true homology between these groups of organisms. In addition, all embryological material is in contradiction with this hypothesis.

A. V. Ivanov in 1968 published the book “ Origin of multicellular animals ”, written on the basis of an analysis of a large amount of factual material and a critical review of literature data. He comes to the conclusion that the most convincing hypothesis of the origin of multicellular organisms is the phagocytella hypothesis of II Mechnikov.

The ancestors of multicellular (Metazoa), apparently, were heterotrophic collar flagellates (Craspedomonadina) from the order Protomonadida (Protomonadida). From a spherical free-swimming colony, consisting of identical flagellates, more complex colonies with greater integration arose. Initially, reproduction was asexual, the colony disintegrated into individual cells, which then turned into new colonies. The emergence of the sexual process led to the division of the cells of the colony into somatic and reproductive. At the same time, the anteroposterior axis of the colony was differentiated and its anterior and posterior ends (poles) were determined. The radial symmetry of the colony acquired a multibeam character.

At first, the sex cells - gametes - were the same and isogamic copulation was observed, and later there was a differentiation of male and female gametes and anisogamy occurred. The fertilized egg - the zygote - began to divide intensively before since then until a new colony like blastula emerged.

Further differentiation of the colony led to its transformation into an independent organism, similar to a phagocytella. At the same time, at first there was a temporary, or optional, and then a permanent isolation of the outer layer, or the kinoblast, and the inner, or phagocytoblast (according to I.I.Mechnikov). The resulting organism, the phagocytella, reproduced both sexually and asexually. The first embryonic stage of development led to the formation of a single-layered free larva. The second stage was postembryonic development, which consisted in the growth of the animal and further differentiation of its cells. In this case, part of the cells left the surface of the larva inward, forming the inner layer... Thus, there was a two-layer phagocytella. Then from the somatic germ cells were separated and the body's sexual maturity arose.

A further stage of development A. V. Ivanov assumes the formation of the mouth opening at the posterior pole of the phagocytella. At first, her amoeboid phagocytes approached the surface anywhere and captured food particles. However, with the appearance of the anterior end of the body, the coordinated beating of the cilia of the kinoblast created an accumulation (concentration) of food particles at the posterior end of the body, in the so-called dead space. Here, a mouth opening appears, through which it is easier for phagocytes to capture food. This circumstance is consistent with the factual material and explains the formation of the primary mouth in all multicellular organisms at the posterior, vegetative pole of the embryo.

A. V. Ivanov considers sponges and intestinal turbellaria to be the closest forms to the original common ancestor of all Metazoa - phagocytella.

The sponges became sedentary and separated very early from the common trunk of Metazoa. Their ancestors were, apparently, organisms similar to phagocytella, which did not yet have either a mouth or intestines. The surface layer of cells (kinoblast) sank inside and began to perform a water-propulsion function instead of a motor one, and the inner layer became external. This is how the well-known eversion of the layers of the body at the lips took place. However, their free-swimming parenchymal larvae are very similar to the larvae of lower multicellular organisms - the planula - and to the hypothetical early phagocytella.

The second branch of development goes to the common ancestor of two-layer animals, from which two types later evolved - coelenterates (Coelenterata) and ctenophores (Ctenophora). In the beginning, these forms were floating. The attached way of life led to the formation of primitive coelenterates, close to hydroid polyps, from which corals and swimming jellyfish later arose. Ctenophores can be considered direct descendants of primitive floating bilayers, which retained the primary mode of movement due to the cilia of the rowing plates, homologous to the kinoblast of the phagocytella.

The third branch of development from phagocytella goes to intestinal turbellaria. Their formation is associated with the transition to a creeping lifestyle, which contributed to the emergence of bilateral symmetry, the formation of the anterior and posterior ends of the body, and the formation of the mouth. The latter arises initially at the posterior end of the body, and then moves to the ventral side.

Thus, according to A.V. Ivanov's theory, ciliary worms are turbellaria, on the one hand, and the primitive ancestors of coelenterates and ctenophores, on the other hand, depart almost simultaneously from the late phagocytella, which already had a mouth, but the phagocytoblast had not yet epithelized ( the intestine has not yet formed). In the future, these groups develop to some extent in parallel.

B. N. Beklemishev, who also shares the hypothesis of II Mechnikov's phagocytella, draws attention to the great similarity (in the main features of organization) of adult ctenophores and turbellaria with coelenterates. He explains this similarity by the common origin of both groups (ctenophores and turbellaria) from more or less close ancestors. According to V.N.Beklemishev, ctenophores and turbellaria have a shorter life cycle in comparison with coelenterates. He suggests that ctenophores and turbellaria evolved from the ancestors of coelenterates by neoteny, that is, from their larval forms, which passed on to progressive evolution. Further development These groups went to some extent in parallel, or convergently, which is manifested in the similarity of the structure (symmetry) of the nervous system and in the formation of an aboral statocyst. However, ctenophores formed as planktonic (with rare exceptions) forms, and turbellarians - as benthic forms. As a result of crawling along the bottom, they developed bilateral symmetry.

The problem of the origin of multicellular and phylogenetic relationships between lower multicellular organisms - sponges, coelenterates, ctenophores, and turbellaria - is very complex. It cannot be considered completely resolved. This requires new data on comparative cytology, embryology, physiology of these groups, using the latest research methods, such as electron microscopy, etc.

Origin of multicellular animals

The problem of the origin of multicellular animals is of interest not only for zoology, but also has great general biological significance. Multicellularity represents the morpho-anatomical basis on which a colossal variety of structural plans, life forms and evolutionary potencies are formed. Thus, knowledge of the ways and reasons for the formation of multicellularity in animals is the key to understanding many important zoological and general biological issues.

Due to its exceptional importance, the problem of the origin of multicellular animals has long attracted the attention of researchers. On this occasion, many hypotheses have been put forward, most of which are currently of historical interest, as interesting examples of the formation of zoological thought. All of these hypotheses are grouped into four categories.

First group make up hypotheses suggesting an independent origin of protozoa and multicellular animals. These include the representations of S. Averntsev (1910) and A.A. Zavarzin (1945). According to these authors, even at the dawn of life on Earth, the primary living substance (primordial mucus), which did not yet have a cellular structure, in one case acquired the organization of protozoa, in the other - immediately a multicellular structure. Such assumptions contradict both the fundamental general biological generalization - the cell theory, and the comparative cytological data indicating the exceptional similarity of the fine cellular structures of Protozoa and Metazoa, which could hardly have arisen independently.

Second group represented by hypotheses that derive multicellular organisms directly from solitary protozoa. In particular, G. Iering (1877), A.A. Tikhomirov (1887), I. Khadzhi (1944), O. Steinbock (1963) and other authors. The essence of these hypotheses is that multicellular animals evolved from large, highly developed and complexly organized protozoa by the so-called cellulization, i.e. a one-time division of the protozoan body into many specialized cells.

Such an assumption, despite all its fantastic character from modern positions, has certain comparative anatomical and embryological grounds. So, some ciliates in terms of the complexity of their organization, at least, are not inferior to lower multicellular ones, such as intestinal turbellaria. Cellularization hypotheses are based on the fact that highly specialized structures of ciliates gave rise to specialized tissues and organs of multicellular animals.

The embryological basis for the cellulization hypothesis is the surface crushing of arthropod eggs. With this type of cleavage, nuclear fission is not at first accompanied by division of the cytoplasm. Cell boundaries appear simultaneously and relatively late.

The hypotheses of cellulization were strongly criticized by V.A. Dogel, V.N. Beklemisheva, A.A. Zakhvatkina, A.V. Ivanova, O. M. Ivanova-Kazas and other major zoologists. The essence of this criticism, in short, is as follows.

First, its authors pointed out the inconsistency of the embryological argument. The fact is that arthropods are animals far removed from the origins of multicellularity, and therefore could hardly preserve primitive forms of development. Arthropod egg crushing is undoubtedly the result of far-reaching specialization. The lower multicellular organisms have a completely different course of ontogenesis.

In addition, based on the hypothesis of cellulization, in the ontogeny of a multicellular animal, all tissues would have to differentiate immediately after syncytial cleavage, and right there. In reality, in the course of the individual development of multicellular organisms (during gastrulation and organogenesis), there is a consistent differentiation and significant movements of cell masses.

Second, highly organized protozoa are too specialized creatures to give rise to animals with a fundamentally different type of organization. Such an assumption contradicts one of the fundamental laws of evolution, which says that at the origins of an evolutionarily young group of organisms is always not the most perfect representative from among the evolutionary predecessors.

Thirdly, the hypothesis of cellulization has no ecological justification. In this regard, the division of the protozoan body into cells seems to be unreasonable.

Third group hypothesis deduces multicellular from colonial protozoa.

Among them, historically, the first hypothesis was gastrea the famous German zoologist Ernst Haeckel (1874), on a long period which has won great popularity among specialists. E. Haeckel based this hypothesis on the fact that all multicellular animals in their development necessarily go through a two-layer stage - gastrula. Based on the Haeckel-Müller biogenetic law (ontogeny is a brief repetition of phylogenesis), Haeckel suggested that each stage of the individual development of a multicellular animal repeats (recapitulates) the corresponding stage of the ancestral form. So, the stage of the zygote in phylogeny corresponds to the stage of a unicellular organism, the stage morula 1 (the late stage of cleavage in the form of a dense rudiment) is responsible sea- a colony of protozoa in the form of a globular cluster, the blastula stage - a colony of protozoa in the form of a hollow sphere, similar to the modern Volvox - blastea... The invagination (invagination) of a part of the wall of a spherical colony, according to Haeckel, led to the formation of a two-layered animal - gastrea corresponding to the stage of invagination gastrula. The outer layer of gastric cells (ectoderm) played the role of the skin, the inner layer (endoderm) played the role of the intestine. The blastopore acted as a mouth opening (Fig. 1). Among modern Metazoa, the most primitive representatives of coelenterates are closest to Haeckel's gastritis, which Haeckel based on the entire phylogenetic tree of multicellular animals.

Rice. 1. The origin of multicellular animals according to Haeckel.

Haeckel's gastric hypothesis was of great historical importance, contributing to the approval of the evolutionary idea and the collapse of Cuvier's "theory of types". However, it was not free from a number of fundamental shortcomings. These include, first of all, the absence of any intelligible ecological and physiological substantiation of the invagination process.

Haeckel's gastric hypothesis was not left alone. Obeying the call of fashion, many scientists have proposed original hypotheses of the colonial origin of multicellular animals. Among them I will mention Lancaster with his "planula theory" (1877) and Bütschli with his "plakula theory" (1884). These views are currently of only highly specialized historical interest, so we will not dwell on them specifically.

A very detailed criticism of Haeckel's theory of gastrea was given by I.I. Mechnikov (1886). Thus, he convincingly demonstrated that intussusception could not be historically the first way to form a two-layer organization of multicellular organisms. The fact is that a completely different mechanism of gastrulation is inherent in primitive multicellular organisms, namely multipolar immigration... Phylogenetic intussusception appeared much later, as a result of the progressive evolution of the ontogeny of multicellular organisms. In addition, the data of comparative physiology unambiguously indicate the secondary nature of cavity digestion, which was preceded by intracellular digestion. Consequently, according to I.I. Mechnikov, it is unlikely that primary multicellular organisms, like Haeckel's gastrea, could have an intestine and a mouth opening.

As an alternative to Haeckel's hypothesis, I.I. Mechnikov proposed an original theory called phagocytella theory(1886). It is reasonably well substantiated and, in a somewhat modernized form, retains its significance at the present time.

When developing the theory of phagocytella I.I. Mechnikov proceeded from the following considerations.

The ancestors of multicellular animals could be unicellular animals with an animal type of nutrition, that is, representatives of the subkingdom Protozoa.

Many flagellates under certain conditions (in particular, during the capture of food) can take the amoeboid form.

Intracellular digestion in the course of evolution arose earlier than cavity digestion, therefore, primary multicellular organisms hardly had an intestine, as well as a mouth opening.

The most primitive methods of gastrulation are multipolar immigration and mixed delamination; the phylogenetic pathways of the formation of a two-layer organization should have been similar.

The initial stage of development of multicellular animals I.I. Mechnikov believed a spherical colony of flagellates, all of which were located at its surface in one layer. Flagella served for swimming of the colony and facilitated the capture of food particles, organizing eddies (the so-called sedimentation method of feeding). The cells that captured the food particle discarded the flagellum, took an amoeboid form and rushed into the colony, where they indulged in the digestion of food. After completing digestion and getting hungry, they rebuilt the flagellum and returned to the surface.

Subsequently, according to I.I. Mechnikov, the initially homogeneous individuals of the colony were divided into two layers - kinoblast with locomotor function, and phagocytoblast with trophic function. This hypothetical organism resembled the larvae of lower multicellular organisms known as parenchymules. Therefore, I.I. Mechnikov called this creature, according to the prevailing zoological tradition, parenchymella... However, bearing in mind the fact that the parenchymula is a purely disseminating stage and does not feed on its own, he changed his mind and suggested a different name - phagocytella(thereby emphasizing the intracellular digestion of this animal).

The theory of I.I. Mechnikova, finding herself in the shadow of Haeckel's gastrea hypothesis, did not receive due recognition, and then was completely forgotten. Only half a century later, it was restored to its rights thanks to the works of V.N. Beklemisheva, A.V. Ivanova, A.A. Zakhvatkina and A.A. Zavarzin. At present, the ideas of I.I. Mechnikov are at the heart of the generally accepted ideas about the origin of multicellular animals, which will be discussed below.

Fourth group hypothesis assumes the origin of multicellular animals from multicellular plants. Franz (1919, 1924) and Hardy (1953) ventured to formulate such views.

So, Franz suggested that multicellular animals come from brown algae, namely from fucus. The main similarities, which Franz attached absolute phylogenetic significance, are the similarity of life cycles and more or less the same nature of sexual reproduction.

Hardy's (1953) concept is as follows. In the opinion of its author, the transition to a multicellular state in plants is easier than in animals, since the general nature of the nutrition of a multicellular plant - the absorption of food by the entire surface of the body - remains the same. The animal, on the other hand, must develop a new way of feeding, which makes the transition to a multicellular state extremely difficult. Otherwise, multicellularity does not give the animal any advantages.

Based on these considerations, Hardy suggested that multicellular animals evolved from already formed metaphytes, which overcame the above difficulties. Lacking mineral nutrition, they began to feed on small organisms, just as modern carnivorous plants do. As a result of this reasoning, Hardy developed a simple polypoid metazoon with a vesicular cavity and tentacles.

The hypotheses of the origin of metazoi from metaphytes are so exotic that it makes no sense to dwell on their critical analysis.

Modern concepts of the origin of multicellular animals

Modern ideas about the origin of multicellular animals are based on the phagocytella hypothesis of I.I. Mechnikov, somewhat modernized and supplemented taking into account later discoveries and ideas.

Before attempting to reconstruct the course of this process, one should think: why, in fact, did animals suddenly need this multicellularity? Have they existed on Earth for a billion years, improving within the framework of a single-celled organization, and suddenly started to create a "state of cells"?

Experts are well aware that any development reaches a qualitatively new level if and only if the possibilities for development within the framework of the old quality are exhausted. In other words, when development runs into a certain “ceiling” that cannot be overcome on the basis of the previous organization. This means that a single-celled being has some fundamental limitations that prevent him from improving.

The analysis of the zoological material made it possible to establish that such restrictions include, first of all, some allometric dependences. It is known that the evolution of life on Earth follows the path of complication, one of the manifestations of which is the so-called phylogenetic growth - a sequential increase in the size of organisms as their phylogenetic development.

In unicellular organisms, this growth is associated with many factors. First of all, with the need to move the elementary relative to the environment. The fact is that the supply of oxygen to the protozoa and the removal of their waste products occurs by diffusion. As a result, a single-celled creature very quickly creates a "desert" around itself, moreover, polluted by its own secretions. Therefore, it is vitally important for him to change the environment, i.e. move from the damaged point "A" to the fresh point "B". However, a small organism has a colossal surface-to-volume ratio and, therefore, suffers greatly from friction against water. In other words, during active movement, it experiences extremely high resistance from the environment. Moreover, this resistance is proportional to the area of ​​the protozoan, and its locomotor power is proportional to its volume. Thus, an increase in the linear dimensions of the simplest, say, twice will lead to the fact that the swimming resistance will increase four times, and the power - eight times. Or, which is the same, specific power(the ratio of power to friction forces) will double! As a result, the protozoan tends to increase in size as an evolutionary response to the need for vigorous swimming.

Another reason for the increase in size is to create in your body a supply of nutrients and reserve biomass, which makes them relatively independent from fluctuations in life resources.

And, finally, the third (but not the last!) Reason - phylogenetic growth is a simple consequence of the increasing complexity of the organization. When there are many different morphological structures, they need a more capacious "container".

Thus, in the course of progressive evolution, the protozoa must increase in size. Can this process go on forever? And why do we not have the right to expect the appearance of, say, an intricately arranged unicellular size of an elephant?

The fact is that the functioning of a unicellular being, like any other living organism, is based on expedient responses to environmental challenges. In unicellular organisms, such reactions are controlled by the nucleus. For example, a substance appeared in the environment. This substance binds to receptors on the outer surface of the cell membrane, and as a result of this interaction, the receptor sends a chemical signal to the nucleus in the form of a molecule. This molecule reaches the nucleus and causes the desired gene to be expressed. As a result, the cell begins to synthesize the required substance: the answer has taken place.

With an increase in the size of the simplest, the distance between the cell membrane and the nucleus increases. The reaction time of the organism to external signals also increases, and it, in the end, begins to be hopelessly delayed, becoming like a very phlegmatic and clumsy giant, defenseless in a rapidly changing environment.

I must say that wildlife has faced a similar problem more than once. A relatively recent example: the appearance on Earth of large dinosaurs with a length from nose to tip of tail over 20 meters. Considering that the speed of nerve impulse conduction in reptiles is of the same order of magnitude (30-40 meters per second), one can imagine how a small but impudent predator deigned to dine on the tail of a dinosaur before he began to realize that in his rear that something happens. There is an opinion that it was for these reasons that the brain of the giants was no larger than a tennis ball, while the bulk of the nerve mass was located in the sacral region. This "invention" reduced the "shoulder" of the reflex arc by about half, reducing by the same number of times the time for a dinosaur to "comprehend" the events that occur with its tail.

What way out of this situation did the protozoa find? This way out consisted in polyenergy: the simplest developed many nuclei, each of which controlled its own "province" - the adjacent area of ​​the cytoplasm.

However, this solution turned out to be only a half measure, since the integrity of the resulting organism was small. The simplest was divided into many "autonomies", and its coordinated control as a whole was hampered by the same distances between the cell membrane and the deep-lying parts of the cell. In this respect, the simplest was likened to the huge and clumsy Russian Empire of the middle of the 19th century, when orders from the capital, which at that time was St. Petersburg, were transmitted to remote provinces by equestrian relay. Is it any wonder that with such a formulation of the case, the governor of Kamchatka learned Crimean war only three months after its start, and even then he received this news not from St. Petersburg, but from the British squadron, which began to bombard Petropavlovsk-Kamchatsky from the sea raid.

Thus, the general plan of the structure of the simplest turned out to be fraught with fundamental limitations that cannot be overcome within the framework of a single-celled organization. Nevertheless, the main line of evolution of protozoa from primitive mononuclear diploid forms to polyploid and, further, polyenergy representatives of the subkingdom is quite rightly interpreted by zoologists as tendency towards multicellularity.

As long as there are problems that cannot be resolved within the framework of a single-celled organization, there remains one way - cooperation unicellular individuals. It is these considerations that underlie modern views on the occurrence of multicellularity in animals.

According to these ideas, the ancestors of multicellular animals were rather primitive flagellates, similar to the modern representatives of Choanophlagellata - collared flagellates. Their phylogenetic closeness is indicated by similarities in the ultrafine structure of the flagellum and kinetosome, mitochondria, the composition of reserve nutrients, as well as the presence of collar cells or cells with collar rudiments in some multicellular animals. In addition, modern Choanophlagellata show a distinct colonial tendency.

The first stage on the way to multicellularity was the unification of single collar flagellates into a simply arranged colony of the type Sphaeroeca- a spherical aggregate of cells oriented outward with their flagella (Fig. 2). The cells of the colony were exactly the same morphologically and functionally. The most that such a colony could be capable of in terms of cell differentiation is the emergence of a morphological gradient, as is the case in modern Volvox - at its functionally anterior pole, the cells are smaller, and gradually increase towards the functionally posterior pole.

Rice. 2. The origin of multicellular animals according to Ivanov.

What prompted single-celled individuals to form a colony? Apparently, the need to overcome the very allometric limitations that prevent protozoa from swimming. The surface of a spherical colony and, consequently, its friction against water is much less than the total surface of its constituent individuals, and the locomotor power of the colony is equal to the sum of the locomotor powers of the individuals. Thus, the cooperation of the protozoa increased the efficiency of swimming.

At first, such a colony reproduced, apparently, only asexually, breaking up into individual cells, each of which gave rise to a new colony (as it happens in modern Sphaeroeca). In order for the colony to develop as a whole, the first differentiation of cells into sexual and somatic cells had to occur. More precisely, in the developmental cycle of the ancestral form, a new generation, represented by sex individuals, should have appeared, similar to what is observed in the colonies of modern Volvox or Proterospongia. Specialized germ cells saved the colony from constant destruction, as they took over the function of reproduction. The colony received the opportunity to progress as a single entity, its integration could intensify from generation to generation and subjugate the individuality of individual individuals.

Second stage- facultative differentiation of the colony into functional groups of cells. The reason for the onset of this stage is the continuing increase in the size of the colony, due to which its constituent cells dispersed along the periphery, and a free space was formed inside, filled with a gelatinous mass. Since the flagellar cells were located on surface colonies, the locomotor power became proportional to the area, and further improvement of locomotion due to a simple increase in size turned out to be impossible - in this respect, evolution has come to a standstill. But the members of the colony had the opportunity to alternate different phases of activity, optimizing the performance of one or another function in turn. So, the cells located outside performed a locomotor function in the interests of the entire colony, and ate one by one, filtering food particles out of the water, each for himself. Having been "loaded" with food, the cells lost their flagellum, acquired an amoeboid shape and went inside the colony, where they concentrated on digesting food. Hungry again, the cells returned to the surface, rebuilt the flagellum, and everything started all over again.

Third stage... This physiological isolation of the cell layers was an important prerequisite for the constant morphological differentiation of the colony. In the end, the cell mass of the colony was divided into two layers, each of which specialized in performing certain functions. The cells of the outer layer - the kinoblast - took over the function of locomotion and, in part, the capture of food (with its subsequent transfer to the cells of the inner layer). The cells of the inner layer - the phagocytoblast - took over the trophic function - the capture of food from the surface of the colony and its digestion (with the subsequent transfer of easily digestible products of digestion to the cells of the kinoblast). Thus, the cells no longer needed to replace each other in their movement from the surface of the colony inward and back, while changing the shape of the flagellate to the shape of an amoeba and the shape of an amoeba to the shape of a flagellate.

Thus, the constant differentiation of the members of the colony into two cell layers made it possible to save time and vital resources of the members of the colony, turned out to be evolutionarily advantageous and was genetically fixed. This is how the first primary multicellular animal(Prometazoa), named early phagocytella, or phagocytella-1.

Fourth stage- the appearance of the first true multicellular animal Eumetazoa. Its essence consisted in the epithelization of the kinoblast, which entailed a series of important evolutionary consequences. The very same epithelialization was caused, first of all, by the need to increase the strength of the intercellular connections of a sufficiently large and actively swimming creature. Thus, the kinoblast cells were reliably "sewn" to each other and formed the very first tissue - ectoderm.

This entailed the following transformations.

1. During the epithelialization of the kinoblast, some of the cells specialized in the sensitive function and the function of carrying out stimulation. This is how the first sensory-nervous elements appeared, forming the primary nervous plexus, or diffuse type nervous system. Sensitive elements concentrated on the aboral pole, where they formed parietal plate... In the end, the phagocytella very early arose a coordination center, on the basis of which it developed primary brain(possibly associated with a statocyst). Due to this, the integration of the phagocytella as a whole organism has increased dramatically.

2. The cells of the phagocytoblast could no longer push their pseudopodia between the flagellate cells, so an opening appeared in the ectoderm — the blastopore, or the primary mouth, through which the phagocytoblast cells could capture food particles. The mouth opening appeared at the functionally posterior pole, since during the swimming of the phagocytella, due to hydrodynamic reasons, it was there that food particles were concentrated. The latter is proved by laboratory experiments with the larvae of some lower multicellular animals recapitulating the late phagocytella: when the carcass particles are added to the water, they all end up in the region of the posterior pole of the larva, where they are phagocytized by the cells of the digestive parenchyma.

3.With the advent of the oral opening, the functionally posterior pole became morphologically posterior and was called oral (or vegetative). In accordance with this, the opposite pole began to be called aboral (or animal), and the axis connecting them - the primary the main axis of the body, a very important coordinate with which the topographic position of organs and parts of all multicellular animals is correlated in morphoanatomical analysis. Thus, a creature arose with radial heteropolar symmetry - the primary form of symmetry of true multicellular organisms.

This creature was named late phagocytella, or phagocytella-2... It is this that underlies the phylogenetic tree of all true multicellular animals of Eumetazoa.

1 Morula translated from Latin means mulberry.

The development of all multicellular animals begins with one cell(zygote or ovum). This well-known fact in the light of the biogenetic law can be considered as proof that the distant ancestors of these animals were unicellular protozoa. The presence of stages of morula and blastula, consisting of identical cells, shows that the further stages of evolutionary development (phylogenesis) of the ancestors of multicellular animals were colonies of protozoa, first in the form of a bunch of cells, and then spherical, which received the name blasts for their resemblance to blastula. In blasteas, which were planktonic organisms, all cells were on the surface, forming a single layer. There were no differences between these cells, and each of them performed all vital functions (ensured the movement of the colony, digested captured food, etc.).
Colonial protozoa, the ancestors of multicellular animals, probably belonged to the flagellate class. Ciliates could not be the ancestors of the animals in question, since the latter never have two types of nuclei, large and small), as is typical for all ciliates. Sporozoans, all without exception leading a parasitic lifestyle, could not give rise to more complex animals, because parasitism leads to a simplification of the organization, and not to its complication. Pseudopods are very slow animals, their evolution was marked by the development of protective devices (shells), and not by the activation of the way of life.
What was the path from spherical, single-layered blasts to the most primitive multicellular animals, which already had a division of functions between groups of cells. The most convincing solution to this question was given by the outstanding Russian scientist I.I.Mechnikov on the basis of his own research. embryonic development lower multicellular animals. In short, the essence of Mechnikov's theory is as follows. Individual cells of the blasts captured food and temporarily submerged inside the colony, where they were digesting food, and then returned to the surface. The assumption about the temporary migration of cells is confirmed by observations of lower multicellular animals, especially sponges, in which the cells are often very loosely connected and can change their position in the body.
During the migration of cells in the blasts, a division of functions is outlined, but only temporary: the same cells, being on the surface, perform one function, and inside the blasts, others. This separation of functions is not enough. The cells that provide the movement of the colony must have a dense membrane and cords, and the cells that capture food with the help of pseudopods must have a thin membrane. Therefore, in the struggle for existence, those descendants of blasts survived, in which the division of functions between β-cells was fixed: some cells constantly remained on the surface and provided communication with the external environment, movement and protection of the body, other cells were constantly inside and served only for digesting food. The connection of cells at this stage of development of multicellular animals was loose and there were many pores in the outer layer through which various microorganisms and organic pieces could get inside, where they were captured by the cells located there with pseudopods. The described hypothetical (putative) forms, consisting of two layers of cells, but not yet having an intestinal cavity, Mechnikov suggested calling parenchymella, or phagocytellae... The first name means filling the middle of the body of these forms with cells, the second - devouring by the internal cells of microorganisms and organic pieces, the outer layer of parenchymella, mainly serving for movement, Mechnikov called kinoblastom, and the internal one serves for the digestion of food, - phagocytoblastoma.
Parenchymella, which already had a division of functions between cells, were no longer colonies of protozoa, but multicellular organisms, albeit very simple ones. The fact that such animals once existed is confirmed, firstly, by data on the development of a number of sponges and coelenterates (migration of many cells at the blastula stage inward and the formation of the parenchymal stage), and secondly, by the structure of one group of lower worms from the type of flatworms , in which there is no intestinal cavity, and the mouth leads to the middle of the body, filled with cells that digest food.
From the parenchymella, according to the hypothesis, there were two-layer animals with an intestinal cavity, where a single opening led - the mouth. The appearance of this cavity was beneficial, since it could serve as a place for the accumulation of food. The intestinal cavity could develop as a result of the gradual expansion of phagocytoblast cells. At first, the cavity was probably small, but its size gradually increased, since animals with a large intestinal cavity could capture larger prey. The possible existence of such animals is confirmed by the wide distribution of the gastrula stage in a wide variety of multicellular animals, and therefore they received the name gastrea. The latter were more complex than the parenchymella and subsequently, in connection with the activation of the lifestyle, primitive nerve cells and muscle fibers appeared in them.
The German scientist E. Haeckel believed that gastritis originated directly from blasts, in which a layer of cells invaded and immediately formed two layers and an intestinal cavity. Putting forward this hypothesis of gastritis, he proceeded from the fact that gastrula during embryonic development in a number of groups of multicellular animals is formed from blastula by invagination (invagination). However, Haeckel did not take into account that this method of formation of gastrula from blastula arose in more developed multicellular animals as a result of reduction and simplification of embryonic development, while in lower multicellular animals the gastrula stage is preceded by a two-layer stage without a cavity (parenchymula).
II Mechnikov, when developing his theory of the origin of multicellular animals, widely used facts indicating the ability of various cells of an animal organism to devour food and other pieces. Based on various observations and experiments, I.I.Mechnikov showed the wide distribution of these phenomena in various groups of the animal world and created phagocytosis theory... According to this theory, phagocytes play a huge role in protecting the animal organism from infectious diseases, which devour bacteria or contribute to their death by releasing special toxic substances. Thus, the theoretical studies of the outstanding Russian zoologist-embryologist on the origin of Metazoa are closely related to the creation of the theory of phagocytosis, which has great importance for medicine and veterinary medicine.

Hypotheses of the origin of multicellular animals

Gastrea hypothesis E. Haeckel (1874).

A transitional form between unicellular and multicellular animals is a single-layered spherical colony of flagellates - "blastea", reminiscent of blastula.

In the process of evolution from the "blastea" by invading the wall of the colony, the first multicellular organisms occur - "gastrea", from which coelenterates and other groups of multicellular organisms originate. E. Haeckel considered the presence of stages of blastula and gastrula on early stages ontogenesis of modern multicellular organisms.

Plakula hypothesis O. Bütschli (1884) is a variant of Haeckel's gastrea hypothesis. Unlike E. Haeckel, this scientist takes a lamellar single-layer colony as a transitional form between unicellular and multicellular animals.

Phagocytella hypothesis I.I. Mechnikov (1882), who discovered the phenomenon of phagocytosis and considered this method of food digestion to be more primitive than cavity digestion. Studying the ontogeny of primitive multicellular sponges, he discovered that gastrula in sponges is formed not by invagination of the blastula, but by immigration of some cells of the outer layer into the embryonic cavity.

These two discoveries were the basis for this hypothesis. The prototype, or living model of the hypothetical ancestor of multicellular organisms - "phagocytella" - I.I. Mechnikov counted the sponge larva - parenchymula.

The main hypotheses are given in rice. 2.1.

Rice. 2.1. Diagram of origin of multicellular (colonial):

I - according to Haeckel's hypothesis, II - according to Bütschli's hypothesis, III - according to Mechnikov's hypothesis

2.2. Sponge Type. Rod Badiaga (Spongilla)

Characteristic of the Sponge type

Sedentary multicellular colonial animals of several hundred individuals, each 1-2 mm in size.

Features of the external structure

There are no real tissues and organs, the body consists of cellular elements.

body shape in the form of a bag, inside there is a cavity, the walls of which are penetrated by numerous small pores

color yellowish brown or greenish, depends on symbiotic organisms - unicellular green algae zoochlorella with which sponges have a symbiotic relationship

Features of the internal structure

Outside, the body of the sponge is covered with flat cells - pinacocytes, and the body cavity is lined with cells with flagella - choanocytes.

Between the layer of pinacocytes and the layer of choanocytes there is a structureless substance - mesoglea.

It contains a variety of cellular elements: pigment cells giving the sponges color; amoebocytes- amoeboid cells that carry nutrients from choanocytes to all cells of the body.

TO olenecites- star-shaped cells that perform connective tissue and support functions; germ cells from which gametes are formed; archaeocytes- cells capable of transforming into all other types of cells; scleroblasts- cells that form skeleton needles.

Sponge skeleton needles - spicules form the skeleton of a sponge (calcareous, horny, silicon, etc.). The skeleton serves as sponges to maintain the shape of the body and to protect against attack (fig. 2.2.)

Oxygen, dissolved in water, and obtained from the cells of symbiont algae.

Organic particles suspended in water, small unicellular algae entering through the pores with water flow, food particles brought by water (filtration). Choanocytes are able to capture pieces of food with flagella and digest them intracellularly.

Amebocytes- amoeboid cells that carry nutrients from choanocytes to all other cells of the body. Sponges extract organics and silicon compounds from the water passing through them, which they use to build spicules.

Sponges have no special excretory organs. Waste products are removed independently by each cell, and large particles that enter the body of the sponge with the flow of water.

Nervous system

The nervous system is absent, the internal cavities are lined with choanocytes - special flagellate collar cells

Reproduction (reproductive organ system)

Sponges multiply asexual and sexual way. Asexual reproduction occurs by external budding and leads to the formation of colonies.

Sponge badyaga is dioecious, but there are no external sex differences. Fertilization takes place in the female colony, where spermatozoa are brought in with the flow of water.

As a result of crushing zygotes a mobile larva - parenchymula with flagella. She leaves the maternal colony later settles on the substrate.

After the larva is attached, it begins metamorphosis: outer cells with flagella sink inward, while flat inner cells move outward.

Those. the germ layers change places, so sponges are called animals " turned inside out".

The larva of most sponges corresponds to the hypothetical phagocytella of I. I. Mechnikov.

It can be assumed that the phagocytella switched to a sedentary lifestyle and in this way gave rise to the Sponge type.

Regeneration well expressed

Significance in nature

Positive

Biofilters are involved in biological water purification. They constitute an important link in river ecosystems and play an essential role in their hydrobiological regime. They participate in food chains, as they are the most important consumers of zoo- and phytoplankton, as well as silicon, which is necessary for the construction of the skeleton. Due to the porous structure of their body, sponges are a good refuge for other animals.

Negative

Badyagi can cause damage by settling in water pipes and other hydraulic structures.

Significance in human life

Positive

They are important in medicine. Dried badyags are used as ointments in the treatment of rheumatism and bruises. In cosmetic practice, badyagi masks are used. The therapeutic effect of sponge preparations is based on mechanical irritation of the skin with spicules, irritation is accompanied by blood flow to the diseased part of the body.

Rice. 2.2. Sponge structure

2.3. Type Intestinal (Coelenterata, Radiata)

general characteristics type

One of the oldest groups of multicellular animals, numbering 9 thousand species.

Ecology

These animals are aquatic and are common in all seas and freshwater bodies. These are lower, mainly marine, multicellular animals, attached to the substrate or floating in the water column.

Life forms

Two life forms are characteristic: a sessile sac-shaped polyp and a free-floating disc-shaped jellyfish. Both forms can alternate in the life cycle of the same species. However, some groups of coelenterates do not have a medusoid generation or have lost the life form of a polyp.

Breathing (respiratory system)

Oxygen, dissolved in water, through the entire surface of the body

Nutrition (digestive system)

The digestive system is primitive and consists of a blindly closed intestinal cavity and mouth opening. Digestion of food begins in the intestinal cavity under the action of enzymes, and ends in specialized cells of the endoderm, i.e., the process of digestion is mixed (cavity and intracellular digestion). Undigested food debris is removed through the mouth opening.

Excretion (excretory system)

There are no excretory organs, excretion occurs over the entire surface of the body

Nervous system

Diffuse type nervous system

Origin

From colonial protozoa - flagellates.

Class 1. Hydroid

Hydra view freshwater (typical)

A predatory animal. With the stinging threads of its tentacles, it infects small aquatic animals, paralyzing and swallowing them.

Small, up to 1 cm, brownish-green animal with a cylindrical body. At one end there is a mouth surrounded by a rim of movable tentacles. (fig. 2.3). At the opposite end there is a stem with a sole that serves for attachment to underwater objects.

Ectoderm (outer layer body) consists of several types of cells:

- epithelial-muscle cells (movement)

- intermediate cells (regeneration)

- stinging cells (defense and attack)

- sex cells (sexual reproduction)

- nerve cells (forming a diffuse nervous system)

Endoderm( the inner layer):

Epithelial-muscle cells (defense, movement)

Digestive cells (phagocytosis and food digestion)

Glandular cells (secretion of digestive juices)

Cavity and intracellular digestion.

Reproduction asexual - budding in the warm season, sexual- with the onset of autumn cold weather. Hermaphrodites. Sperm are released into the water and merge with a mature egg in the ectoderm of another hydra. The zygote is surrounded by a protective shell and hibernates at the bottom of the reservoir. In the spring, a young hydra develops from it, which reproduces by budding.

Hydras are able to restore lost body parts due to the multiplication and differentiation of nonspecific (intermediate) cells

Rice. 2.3. Features of the structure and life of the freshwater hydra

Hydroid polyps Hydroidea

Development cycle hydroid polyps consists of alternating two generations, different in structure and method of reproduction (fig. 2.4).

Polyps - the first generation leads a sedentary lifestyle and reproduces only asexually, producing polyps and jellyfish by budding.

Jellyfish(second generation) break away from the colonies of polyps and move to a free mobile way of life (hydromedusa). They reproduce sexually and again give rise to the generation of polyps.

Most Hydroidea have a typical alternation of generations. In some representatives, there is a partial suppression of one of the generations, namely the medusoid. The jellyfish formed on the colony stop breaking away from it and, remaining in place, develop germ cells in themselves.

Such jellyfish, or jellyfish, are characterized by underdevelopment of the mouth, sensory organs and some other organs. The suppression of the jellyfish generation can go even further, and the jellyfish gradually lose their characteristic shape and turn into simple sacs filled with sex cells (gonophores) sitting on a colony of polyps.

Being at first a free-moving independent generation, jellyfish, thus, gradually become, as it were, the genitals of a colony of polyps: an interesting example of reducing an individual to the degree of a simple organ.

Rice. 2.4. Features of the structure and reproduction of colonial hydroid polyps

Class 2. Scyphoid jellyfish

Jellyfish are predators, rare species (deep-sea) feed on dead organisms. The tentacles of jellyfish are equipped with a large number of stinging cells. Many jellyfish burns are sensitive to large animals and humans.

There is a gelatinous between the ectoderm and endoderm mesoglea... At the edges of the umbrella there are tentacles surrounding the mouth, which is on the underside. The mouth leads into the gastric cavity, from which radial canals extend, which form the gastric system. The gastric cavity is divided into chambers.

The mode of movement is "reactive", movement is achieved by reducing the walls of the umbrella.

They have a more developed nervous system with accumulations of nerve cells in the form of nodules - ganglia around the circumference of the bell.

Along the edge of the umbrella are complex senses - ropalia, each of which contains the "olfactory fossa", the organ of balance and stimulation of the movement of the umbrella - statocyst, light-sensitive peephole.

Rice. 2.5. The structure of the jellyfish and polyp

Jellyfish are dioecious. Sex cells are formed in the sex glands - gonads, located in the endoderm.

In the life cycle of jellyfish, sexual and asexual generations alternate naturally. The sex glands are located in the endoderm under the radial canals or on the oral pedicle. The reproductive products are released through the mouth into the sea, fertilization occurs.

A free-living larva develops from the zygote - planula which turns into a small polyp in the spring.

Polyps form groups similar to colonies. Gradually, they diverge and turn into adult jellyfish. (fig. 2.5-2.6).

Rice. 2.6. Cycle of development of scyphomedusa

Class 3. Coral polyps

Coral polyps (6,000 species) are colonial or solitary marine organisms. Coral polyps are larger than hydroid polyps. The body is cylindrical. In colonial forms, the lower end of the polyp's body is attached to the colony, while in single polyps it is equipped with an attachment sole. The tentacles of coral polyps are located in one or more closely spaced corollas (fig. 2.7).

A separate individual of the colony, or the so-called hydrant, in its structure is similar to a hydra. The body of the colony is branched cenosark, inside which there are individual polyps, interconnected by outgrowths of the intestinal cavity into a single digestive system, which allows the food captured by one polyp to be distributed between the members of the colony. Outside cenosark covered with a hard shell.

Colonial polyps reproduce asexually - by budding. At the same time, individuals that have developed on a polyp do not break off, like in a hydra, but remain associated with the mother's body. The adult colony looks like a bush and consists mainly of two types of polyps: gastrozoids(hydrants), which provide foraging and protection of the colony by stinging cells on the tentacles, and gonozoids that are responsible for reproduction. There are also polyps that are specialized for a protective function.

Gonozoid It is an elongated rod-shaped formation with an extension at the top, without a mouth opening and tentacles. Such an individual cannot feed on its own, it receives food from hydrants through the gastric system of the colony. This formation is called blastostyle. The skeletal membrane gives an expansion around the blastostyle - a gonoteca. All this education as a whole is called gonangia. In them, jellyfish are formed by budding. They bud off from the blastostyle, emerge from the gonang and begin to lead a free lifestyle. As the jellyfish grows in its gonads, germ cells are formed, which are released into the external environment, where fertilization takes place.

A blastula is formed from a fertilized egg (zygote), with the further development of which a two-layer larva, which is free floating in water, covered with cilia, is formed - a planula. The planula settles to the bottom, attaches itself to underwater objects and continues to grow and gives rise to a new polyp. This polyp forms a new colony by budding.

Rice. 2.7. Coral polyp structure

Asexual reproduction - by budding, and sexual - with metamorphosis, through the stage of a free-floating larva - planula. For coral polyps, only the polypoid state is characteristic, there is no alternation of generations, since there is no medusoid stage.

The cells of the ectoderm of coral polyps produce horny substance or secrete carbonic lime, from which the external or internal skeleton is built. The skeleton plays a very important role in coral polyps. (fig. 2.8).

Coral reefs are unique ecosystems in which a huge number of other animals find shelter: molluscs, worms, echinoderms, fish.

Rice. 2.8. Reef coral polyps

Eight-rayed corals have a skeleton consisting of individual calcareous needles - spicules, located in the mesoglea. The skeleton plays the role of support and protection.

Among the six-rayed corals, there are skeletal forms - Anemones (fig. 2.9). These are large, solitary coral polyps without a skeleton. They are capable of slow locomotion using a muscular sole. When irritated, they contract strongly, retract the tentacles and turn into a small lump. Large anemones are predators that feed on crayfish, molluscs, etc.

Anemones, settling on empty shells of mollusks, in which hermit crabs live, together with them form an interesting symbiosis: the cancer moves the anemones along the bottom to more favorable places for hunting and delivers food to it - the remains of its meal, while the anemones protect cancer from enemies with the help of stinging cells (fig. 2.10).

Rice. 2.9. Actinia

Rice. 2.10. Symbiosis of anemones with hermit crab

Another species of anemone has a symbiotic relationship with Clown fish... A bright fish, immune to the poison of tentacles, lures enemies, and the anemones grab them and eat them (fig. 2.11).

Rice. 2.11. Clownfish among the tentacles of anemones

Ecology

Chemical and thermal pollution of the ocean, discharge of polluted waters lead to the excessive development of phytoplankton, which intercept sunlight needed by corals. Corals are destroyed by starfish.

A type Flatworms

General characteristics of the type

External structure of the body

There are anterior and posterior ends of the body, dorsal and ventral sides.

body shape flat, in the dorsal-abdominal direction (aromorphosis type)

dimensions from 2 cm to several meters

symmetry bilateral (aromorphosis type)

covers formed by the epithelium (ectodermal origin), to which muscles are attached - longitudinal, annular and oblique, together forming a skin-muscular sac. Muscles are formed from the mesoderm.

Internal structure of the body (Fig. 2.19, 2.21)

Flatworms are three-layered. In the process of ontogenesis, they form not two, as in coelenterates, but three germ layers. Between the ectoderm, which forms the integument, and the endoderm, from which the intestine is built, they have an intermediate germ layer - mesoderm... In the process of evolution, tissues appeared: muscle, connective, epithelial and nervous (type aromorphoses).

Body cavity

Missing. The space between endoderm and ectoderm is filled with parenchyma. The parenchyma is located between all internal organs. .

Digestive system

The mouth is on the ventral side of the body. Intracellular digestion, partly in the intestinal cavity. There is no anal opening, so undigested food debris is removed through the mouth.

Rice. 2.19. Internal structure of flatworms

Circulatory system

Missing

Respiratory system

Excretory system

Protonephridial tubular system... These tubules from the side of the body parenchyma end in a terminal cell with a bundle of cilia, which is called flame cage... On the other hand, the tubules flow into the main canal, which opens outward. The fiery cell filters waste products from the parenchyma, which are removed from the body through the tubule system (aromorphosis type).

Nervous system

The cephalic ganglion, from which two main nerve trunks with numerous nerves branch off (aromorphosis type).

Sense organs

Reproduction

Sexual. Hermaphrodite. Fertilization is internal, cross (aromorphosis type)

Development

Regeneration

In some species

Significance in nature

Positive

They are an important link in the food chain for many animals.

Significance in human life

Many cause significant harm to livestock, causing disease and sometimes death of livestock. Some flatworms cause serious illness in humans. Swimmers and divers damage and break off corals with flippers, and the silt raised by them settles on the corals, which also leads to their death

Taxonomy

Type Flatworms have about 12.5 thousand species. Classes are distinguished in the type:

Class 1. Ciliary worms, or Turbellaria (3 thousand centuries) - all free living in the sea or in fresh water bodies.

Milk Planaria species

Ecology Habitat: fresh water. Predatory animals

Free-swimming aquatic animals with the help of cilia. There are front and rear ends of the body, dorsal and abdominal sides (fig. 2.20). The integument is formed by the ciliary epithelium. Sizes up to 2 cm.

Direct development. Fertilized eggs are deposited in a cocoon where small planaria develop. Regeneration well developed.

Rice. 2.20. Milk planaria

Rice. 2.21. Structural features of milk planaria

Liver fluke species

External structure of the body

body shape flattened, has two suckers - oral and abdominal.

dimensions up to 2-5 cm

Internal structure (fig. 2.22)

Musculocutaneous sac: skin with cuticle and three layers of muscles (diagonal, transverse and longitudinal). The musculocutaneous sac contains internal organs.

Body cavity missing. Filled with parenchyma.

Digestive system

Oral opening with a suction cup passing into the muscular pharynx, esophagus, bifurcated midgut. There is no anal opening, so all undigested food debris is removed through the mouth.

Excretory system

Protonephridial system. The excretory opening in flukes is located at the posterior end of the body.

Nervous system

The periopharyngeal nerve ring, from which 2-3 pairs of nerve trunks depart, connected by nerves.

Sense organs in the form of nerve endings and organs of chemical sense

Reproduction

The reproductive system is very complex, since flukes are hermaphrodites... Fertilization is internal. Self-fertilization is possible.

Rice. 2.22. Internal structure of the hepatic fluke

Development cycle

The hosts in which the larval stages of the fluke develop are intermediate hosts.

Stages of development

1) Eggs fall into environment along with the excrement of the final owner. For further development, the eggs must necessarily get into a fresh water body.

2) In the water, a larva with cilia emerges from the egg miracidium... That swims for a while, then finds an intermediate host of a freshwater gastropod mollusc Small pond snail and is introduced into his liver.

3) In the liver of a mollusc miracidium turns into the next larval stage - sporocyst.

5) Cercariae break through the mollusk and go out into the water.

6) Cercariae swim in water for some time, then attach to plants, lose their tail, become covered with a thick shell and turn into adolescarian.

7) Livestock eats grass with attached adolescaria.

8) In the intestines of the final host, the shell adolescaria dissolves and the liver fluke comes out and enters the liver.

Thus, the only way to infect the final host is by swallowing adolescarian (fig. 2.23).

Rice. 2.23. Development cycle of the hepatic fluke

Species Bull tapeworm

External structure of the body

Development cycle

The larva develops in the eggs - oncosphere with three pairs of hooks at the rear pole, which turns into finnu (cysticercus)... This is the vesicular stage of development of the tapeworm, always located in the body of the intermediate host. Finna is a bottle filled with liquid with a screwed-in head the size of a grain of rice.

Human infection occurs when eating finnose meat that has undergone insufficient heat treatment.

When it enters the human intestine, the head turns out, and the neck begins to produce segments.

Rice. 2.25. The development cycle of a bull tapeworm

General characteristics of the type

Body cavity

The primary body cavity (schizocele), in which the parenchyma was replaced by a liquid under high pressure. The body cavity performs a supporting function (hydroskeleton) and is involved in transport and metabolism within the body (aromorphosis type)

Features of the internal structure (Fig. 2.30)

Three-layer animals. The body is not segmented. Musculocutaneous sac with 4 strands of longitudinal muscles. Roundworms do not have transverse muscles, therefore they cannot shorten and lengthen the body.

Respiratory system

Digestive system

The presence of the mouth, pharynx, stomach, the appearance of the posterior intestine and anus, which made it possible to make the digestion process step by step

Circulatory system

Absent, the transport function is performed by the cavity fluid.

Excretory system

« Cutaneous gland"Represented by secretory cells located in the front of the body. One or two canals extend from it, passing in the lateral ridges of the hypodermis. Behind they are blindly closed, in front they are connected to the excretory duct, which opens excretory occasion... In addition, on the walls of the excretory canals in the front of the body, there are four large phagocytic cells... They capture and accumulate residual metabolic products in the cytoplasm.

Nervous system

In the process of evolution, a concentration of nerve cells occurred, the periopharyngeal nerve ring and 6 abdominal nerve trunks (aromorphosis type) were formed

Sense organs

Reproduction

Dissolved animals. The female is larger than the male. Lays more than 200 thousand eggs per day. The eggs are covered with several dense shells that protect the embryo from adverse environmental conditions.

The emergence of dioeciousness and internal fertilization provided combinative variability and genetic diversity in offspring.

Development cycle

Significance in nature

Significance in human life

Rice. 2.30. Internal structure of roundworms

Class 2. Kinorinha

They live in marine soil, very small worms (less than 1 mm), feed on unicellular algae and microorganisms (fig. 2.32).

Rice. 2.32. Kinorinha

Class 3. Hairy

Class 4. Nematodes

Species Human Ascaris

Roundworms are one of the most common human helminths. They are found in almost all landscape and climatic zones, with the exception of the permafrost zone, high mountains, deserts and semi-deserts. It is most often found among inhabitants of tropical and subtropical belts, where the prevalence of ascaris reaches 60 - 80%. Ascaris has been known to people for a long time, as evidenced by the mention of this helminth in the writings of Hippocrates.

According to the World Health Organization (WHO), about 1 billion people are affected by ascarism in the world. The structure of the body is fusiform, not segmented, round in cross section. At the front end of the body there is a mouth with three lips. Males are 20-25 cm long, females 40 cm. Color yellowish yellow (fig 2.33).

Rice . 2.33. Human roundworm

The musculocutaneous sac consists of cuticle-covered skin with a non-cellular hypoderm underneath. The longitudinal muscles are attached to the skin. Missing. Respiration is anaerobic (glycolysis). Sense organs tactile hillocks and fossa. Sexual reproduction. Dissolved animals. Sexual dimorphism (i.e., males and females are outwardly different). In females, the posterior end is straight, in males it is pointed and bent to the ventral side. Fertilization is internal. They reproduce by eggs. The female roundworm lays more than 200,000 eggs per day.

Development cycle

Development occurs when a person swallows roundworm eggs while eating unwashed vegetables or fruits, or if the rules of personal hygiene are not followed. Development proceeds without a change of owners.

The eggs are covered with several protective shells and are able to remain viable for up to 10 years. Further development should take place in the soil at an optimal temperature of 20-25 degrees, sufficient humidity and access to oxygen, mobile larvae develop in 21-24 days. At temperatures below 12 degrees and above 38, the larvae do not develop. Under favorable conditions, within 15 - 20 days, larvae are formed in the egg, capable of developing in the human body. In the human gastrointestinal tract, they come out of the eggs.

With the blood flow, the larvae enter the liver, and then into the lungs. This is where they develop. The larvae then “cough up” down the throat and are swallowed again. After 2-2.5 months, adult roundworms, capable of fertilization, develop from the larvae (fig. 2.34).

Pathogenic action of larvae: mechanical and toxic-allergic action. Pathogenic action of adults: mechanical, toxic-allergic action, mutagenic, metabolic disorders.

Ascariasis symptoms:With cough, gastrointestinal disturbances, weakness

Rice. 2.34. Ascaris development cycle

Human pinworm

Adult worms are small, females up to 12 mm, males up to 5 mm. Females lay eggs on the skin near the anus, causing itching. Once under the nails, eggs can easily enter the baby's mouth.

Animals are eukaryotic heterotrophic organisms. More than 2.0 million species have been described.

The Kingdom of Animals has a number of distinctive features:

1. Heterotrophic type of food. Most have holozoan, some have osmotrophic, phago- and pinocytosis. Some mixotrophs (euglena green).
2. Specific features in the organization of the animal cell: does not have a cell wall (therefore it can take on a different shape), the vacuole system is not developed, there are centrioles, many cells are equipped with cilia or flagella, the main storage substance is glycogen.
3. Four types of fabrics: epithelial, connective, muscular and nervous.
4. Mostly active lifestyle which is developmental musculoskeletal and nervous systems.
5. Available excretory organs and nitrogen-containing waste products are released (ammonia, urea, uric acid, etc.).
6. The highest are characterized by complex behavioral responses... Highly organized forms are able to carry out processes of higher nervous activity.
7. Most have nervous and humoral regulation system(only humoral in plants).
8. Available protective (immune) system.
9. Diffuse growth(that is, the growth of the entire surface, and not due to certain growth points) and limited.
10. Life cycles are easier than those of plants... The haploid stage is represented only by gametes (with the exception of sporozoans and foraminifera). Reduction division is carried out directly in the process of gametogenesis.
Systematics. The Kingdom of Animals is divided into two subkingdoms: Unicellular and Multicellular.
Subkingdom Unicellular includes types: Sarcomastigophore (classes Sarcodes and Flagellates), Ciliates (class Ciliated ciliates), Apicomplex (class Sporozoa).
Subkingdom Multicellular includes types: Intestinal (classes Hydroid, Scyphoid and Coral polyps), Flatworms (classes Flukes, Tapeworms, Ciliated worms), Roundworms (class Actually roundworms, or Nematodes), Ringworms (classes Small bristles, Polychaetes and Leeches), Molluscs (classes Gastropods, Bivalves, Cephalopods), Arthropods (classes Crustaceans, Arachnids and Insects), Chordates. Type Chordates are divided into three subtypes: tunicates (class Ascidia), Skullless (class Lancelet), Vertebrates (classes Cartilaginous fish, Bony fish, Amphibians (Amphibians), Reptiles (Reptiles), Birds, Mammals).

Subkingdom protozoa (unicellular)

general characteristics

Sarcomastigophora type

Class Roots (Sarcodes)

Infusoria Type

Class Ciliated ciliates

Type intestinal cavity

general characteristics

About 9 thousand species of coelenterates are known. Habitat - aquatic (marine bodies with the exception of a few freshwater species). Lifestyle - free-living: free-floating or attached forms.
Systematics. The Intestinal type includes the classes: Hydroid, Scyphoid and Coral polyps.
Structure. For most coelenterates, two life forms are characteristic: an attached polyp and a free-floating jellyfish. For many, both forms alternate during the life cycle (polyps - asexual generation, jellyfish - sexual).
Polyp(attached form) looks like an elongated bag with an opening - a mouth, which is surrounded by tentacles and leads into the gastric (intestinal) cavity.

The rear end of the body (sole) is fixed to the substrate. Attached forms can be either solitary (hydra) or colonial (coral polyps).
Jellyfish(floating form) has the shape of a bell, umbrella or saucer, under the arch of which there is a mouth, surrounded by mouth lobes. Tentacles are located along the edge of the dome. Floating forms are always solitary.
Body sizes from 1 mm to 2 m. Intestinal cavities have radial (radial) symmetry type, that is, several planes of symmetry can be drawn through the body. This two-layer animals - their development occurs from two germ layers. The body is formed by two layers of cells: outer - ectoderm and internal - endoderm... Between them is a layer of intercellular gelatinous substance - mesoglea(in jellyfish and hydroid polyps) or a supporting plate that serves as an internal skeleton (in coral polyps). Coral polyps and colonial hydroids also have an external calcareous or horny skeleton.
The cells of the ectoderm and endoderm are differentiated according to their functions.
Ectoderm cells. Ectoderm includes epithelial-muscular, stinging, nerve, intermediate and germ cells.
Endoderm cells. The endoderm includes epithelial-muscular, glandular, nerve and germ cells.
Epithelial-muscular cells line the gastric cavity, have 2–5 flagella, muscle fibers (located perpendicular to the longitudinal axis of the body), capable of forming pseudopods. Provide the movement of water in the gastric cavity and intracellular digestion.
Glandular cells produce and secrete digestive enzymes into the intestinal cavity, providing cavity digestion.
Nervous the cells are analogous to the nerve cells of the ectoderm.
Sexual(in scyphomedusa) are similar to the sex cells of the ectoderm.
Motion is carried out due to the contraction of muscle fibers of the epithelial-muscle cells of the outer and inner layers of the body. The contraction of the longitudinal muscle fibers of the ectoderm cells leads to a shortening of the body and tentacles, the contraction of the transverse fibers of the endoderm cells stretches the body in length. Attached forms have the most mobile tentacles. Solitary polyps (hydra) move by "somersault", jellyfish - in a reactive way.
Irritability is possible due to the primitive nervous system of a diffuse type and is carried out in the form of elementary reflexes... For example, in response to a needle prick, the entire body of the hydra contracts. Attached forms of coelenterates do not have developed sense organs, with the exception of touch. Movable forms have organs of vision (eyes) and balance ( statocysts- bags with lime carbonated pebbles inside).
Digestion. Most coelenterates actively capture food with their tentacles. For the attack, stinging cells are used, which paralyze the victim. Food through the mouth enters the digestive (gastric) cavity, where it is digested. There are two types of digestion: intracellular and cavity. Intracellular digestion is carried out by the epithelial-muscle cells of the endoderm, which capture food particles by endocytosis. Cavity digestion is possible thanks to enzymes secreted into the gastric cavity by glandular cells. Undigested residues from the cells are thrown into the cavity, from where they are removed through the mouth with a stream of water.
Breathing and excretion metabolic products are carried out by the entire surface of the body.
Regeneration- restoration of lost or damaged body parts. It is possible due to the multiplication and differentiation of intermediate cells.
Reproduction. Most are dioecious. Some hydroids - hermaphrodites- have both ovaries and testes. The alternation of asexual and sexual reproduction is characteristic. Asexual reproduction is carried out by budding or strobilation. Budding- reproduction by the formation of a kidney on the maternal organism - an outgrowth from which a new individual is formed. Strobilation- reproduction by multiple transverse divisions of the polyp into several parts. In primitive hydroids, fertilization of the egg occurs on the mother's body. Direct development. In jellyfish and marine hydroids, germ cells are released into the water, where fertilization takes place. Development with metamorphosis, larva - planula.
Origin and aromorphosis. The following aromorphoses led to the emergence of the type: cell differentiation and tissue formation, diffuse type nervous system, cavity digestion.
Meaning. Intestinal cavities are an important link in the food chains of marine animals, they contribute to the purification of water (biological filter feeders). Some types of jellyfish are poisonous (cyanea, krestovichok), some are used for food. Coral polyps form unique ecological systems of coral reefs. At the same time, coral reefs and islands (atolls) make navigation difficult. The skeletons of coral polyps form deposits of limestone used in construction.

Class Hydroid

Life forms are polypous (freshwater hydra) or polypous and short-term jellyfish (obelia).
Freshwater hydra. Habitat - fresh water. Free living, attached. The body is about 1 cm long. The body consists of a sac-like body, sole and tentacles. Attaches to the substrate with the sole. The body is two-layered. The mouth is surrounded by tentacles (5–12), which serve to capture food. Ectoderm cells: epithelial-muscular, nervous, stinging, intermediate, reproductive. Endoderm cells: epithelial-muscular, glandular and nervous. With the help of epithelial-muscle cells, the body is able to move. Stinging cells are used for defense and attack. Breathing is carried out over the entire surface of the body. The nervous system is of a diffuse type, consists of nerve cells scattered throughout the body. The sense of touch is developed. The gastric cavity has no septa and channels. Asexual reproduction (budding) occurs in summer. Sexual reproduction takes place in the fall. In the ectoderm, the gonads are formed, where gametes are formed (sperm with flagella and an amoeboid egg), fertilization occurs on the body of the maternal hydra. The medusoid form is absent. Direct development.
Hydroid polyps(obelia). The alternation of asexual and sexual generations (metagenesis) is characteristic. Asexual generation (polyps) forms colonies in the form of a tree or bush. The sexual generation - hydroid jellyfish - are formed by budding as part of a colony, then they separate from it and lead a free lifestyle. Reproduction in hydroid jellyfish is sexual. External insemination (sex cells are released into the water). Development with metamorphosis (larva - planula).

Class Scyphoid

Scyphoid jellyfish s (cornerot, cyanea, gonionema). They live only in the seas. The jellyfish stage prevails over the polyp stage. The jellyfish resembles an inverted and highly flattened polyp. The content of jellyfish is represented by highly developed mesoglea (contains up to 98% water). Along the edge of the umbrella there is an accumulation of nerve cells in the form of ganglia. Sense organs: balance - statocysts, vision - eyes. The intestinal cavity is represented by a system of communicating channels (4 radial and 1 annular). The movement of jellyfish in the water is carried out according to the reactive principle by pushing water out from under the dome while reducing the walls of the umbrella. Split-sex. Alternation of generations is characteristic. The multiplication of the polyp occurs strobiling- ordered transverse division of the polyp into several parts. Sex cells are formed in the endoderm. A larva develops from a fertilized egg. After attachment to the substrate, a polyp develops from it. Growing up, the polyp begins to bud off young jellyfish.

Coral polyps class

Coral polyps(sea anemone, horny coral, red coral). They exist only in the form of a polyp. They live in the shallow waters of the tropical seas. There are solitary (rare) and colonial forms. The mouth is surrounded by either eight tentacles (eight-pointed corals) or a multiple of six (six-pointed corals). They have an external calcareous or horny skeleton, formed from the ectoderm, or an internal skeleton, formed in the mesoglea. In the development cycle, there is no medusoid form and alternation of generations. Asexual reproduction (budding) and sexual. Dissolved, sex cells are formed in the endoderm. Development is direct or with metamorphosis (larva - planula). Calcareous skeletons of colonial forms form reefs and oceanic islands.

Type flatworms

general characteristics

Class Ciliary worms

White (milky) planaria. It feeds on aquatic invertebrates. Reaches a length of 25 mm. The body is flattened, covered with cilia, the posterior end is pointed. At the front end it has small eyes and chemical sense organs. The internal structure is the same as that of all representatives of flatworms.

Fluke class

Class Tapeworms

Bovine tapeworm. Size 4-10 m. Body shape - ribbon-like. Body parts - head, neck, segments(up to 1 thousand and more). The head has four suction cups, the neck is undivided, the body is long, ribbon-like, dissected. The digestive system is absent. The respiratory system is absent. Anaerobe. The nervous system is poorly developed. Chains are hermaphrodites. Each segment has one ovary and many testicles. Segments containing eggs are secreted from the human intestine (the main host). Together with the grass, they enter the cow's stomach (intermediate host). Six-hooked larvae emerge from the eggs, which penetrate into the blood vessels of the intestine and then into the muscles. Here the larvae turn into finns(a vial with a chain head inside). When a person eats uncooked finnose meat, the head of the tapeworm attaches to the wall and begins to produce segments.
Echinococcus. Adult form up to 6 mm long. Consists of 3-4 segments, on the head it has suckers and a proboscis with a rim of hooks. The segments are not separated. The main owner is dogs, wolves, foxes. Their tapeworm lives in the small intestine. Intermediate - sheep, pigs, goats, cattle, deer, humans. In the intermediate host, the Finns stage develops - a bubble with many heads. Bubbles develop in the lungs, liver, brain, bones and are about the size of a child's head. Human infection occurs by swallowing tapeworm eggs that fall on the hands after contact with dogs and wild animals.

Type roundworms

general characteristics

Class of Nematodes (Properly roundworms)

Type annelids

general characteristics

Structure. Bilateral body symmetry. Body sizes from 0.5 mm to 3 m. The body is subdivided into the head lobe, trunk and anal lobe. Polychaetae have a separate head with eyes, tentacles, and antennae. The body is segmented (external and internal segmentation). The body contains from 5 to 800 identical segments in the form of rings.

The segments have the same external and internal structure (metamerism) and perform similar functions. The metameric structure of the body determines a high capacity for regeneration.
The body wall is formed musculocutaneous sac, consisting of a single-layer epithelium covered with a thin cuticle, two layers of smooth muscles: the outer annular and inner longitudinal, and a single-layer epithelium of the secondary body cavity. With the contraction of the annular muscles, the body of the worm becomes long and thin, with the contraction of the longitudinal muscles, it shortens and thickens.
Organs of movement - parapodia(available in polychaetae). These are outgrowths of the musculocutaneous sac on each segment with tufts of bristles. In small bristles, only bundles of bristles are preserved.
Body cavity secondary - the whole(has an epithelial lining that covers the skin-muscular sac from the inside and the organs of the digestive system from the outside). In most representatives, the body cavity is divided by transverse septa, corresponding to the body segments. The cavity fluid is a hydroskeleton and an internal environment; it is involved in the transport of metabolic products, nutrients and reproductive products.
Digestive system consists of three sections: the anterior (mouth, muscular pharynx, esophagus, goiter), middle (tubular stomach and midgut), and posterior (hindgut and anus). The glands of the esophagus and midgut secrete enzymes to digest food. Absorption of nutrients occurs in the midgut.
Circulatory system closed. There are two main vessels: dorsal and abdominal connected in each segment by annular vessels. Through the dorsal vessel, blood moves from the posterior end of the body to the anterior one, along the abdominal end - from front to back. The movement of blood is carried out due to the rhythmic contractions of the walls of the dorsal vessel and the annular vessels ("heart") in the pharynx, which have thick muscular walls. Many have red blood.
Breath. Most annelids have cutaneous respiration. Polychaetae have respiratory organs - cirrus or leaf-shaped gills... These are the modified dorsal antennae of the parapodia or cephalic lobe.
Excretory system metanephridial type. Metanephridia look like tubes with funnels. Two in each segment. The funnel, surrounded by cilia, and the convoluted tubules are located in one segment, and the short tubule, which opens outward with an opening - an excretory pore, is in the adjacent segment.
Nervous system represented by the supraopharyngeal and subopharyngeal nodes ( ganglia), the periopharyngeal nerve ring (connects the supraopharyngeal and subopharyngeal ganglia) and abdominal nerve cord consisting of paired nerve nodes in each segment, connected by longitudinal and transverse nerve trunks.
Sense organs. Polychaetae have organs of balance and vision (2 or 4 eyes). But most have only separate olfactory, tactile, taste and light-sensitive cells.
Reproduction and development. Soil and freshwater forms are mostly hermaphrodites. The sex glands develop only in certain segments. Internal insemination. The type of development is direct. In addition to sexual reproduction, asexual (budding and fragmentation) is also characteristic. Fragmentation is carried out through regeneration - the restoration of lost tissues and body parts. Marine representatives of the type are dioecious. Their sex glands develop in all or in certain segments of the body. Development with metamorphosis, larva - trochophora.
Origin and aromorphosis. The following aromorphoses led to the emergence of the type: organs of movement, respiratory organs, closed circulatory system, secondary body cavity, body segmentation.
Meaning. Earthworms improve the structure and fertility of the soil. The palolo ocean worm is eaten by humans. Medicinal leeches are used for bloodletting.

Class Small-bristle (Oligochaetes)

Representatives: earthworms, tubules, etc. Most of the small bristles live in the soil and fresh waters. Detritophages- feed on semi-decomposed remains of plants and animals. There are no parapodia. The bristles extend directly from the body wall. The cephalic lobe is poorly expressed. The sense organs are often absent, but there are olfactory, tactile, gustatory, light-sensitive cells. Hermaphrodites. Insemination is internal, cross. Development is direct, takes place in a cocoon, which, after fertilization, forms on the body of the worm in the form of a belt, and then slides off it.
The role of earthworms in soil formation is enormous. They contribute to the accumulation of humus and improve the structure of the soil, thereby increasing soil fertility.

Class Polychaetes (Polychaetes)

Leech class

Shellfish type

general characteristics

More than 130 thousand species have been described. In terms of the number of species, molluscs rank second after arthropods. Habitat: sea and fresh water, humid land. Most molluscs are free-living. Protostomes. They develop from three germ layers. Lead a sedentary lifestyle.

Systematics. The type of Molluscs includes the classes: Gastropods, Bivalves, Cephalopods.
Structure. Molluscs (soft-bodied) have a soft, non-segmented body. Most are bilaterally symmetrical, and gastropods are asymmetric. Body sizes from 2-3 mm to 18 m.
Divisions of the body. The body is divided into head, leg, torso

Bivalves have no head. Leg- This is a muscular outgrowth of the abdominal wall of the body, which serves for movement. Torso contains internal organs, on head the mouth and senses are located.
The body of the clam is usually covered with sink... It can be solid, bivalve, lamellar. In some, the shell is reduced (slugs, cephalopods). The shell performs a protective function and the role of an external skeleton. Usually it consists of three layers: outer - organic (horny), middle - calcareous, inner - mother-of-pearl (porcelain). The shell is formed from substances secreted by the mantle. Mantle- a fold of skin completely or partially covering the body of the mollusk.
Between the mantle and the body of the mollusc is mantle cavity... It houses the respiratory and chemical sense organs and opens the digestive, excretory and reproductive systems. The mantle cavity communicates with the external environment siphons(in aquatic forms) or breathing holes(at ground level).
Body cavity secondary, reduced in adulthood. Its remains are the pericardial sac and the cavities of the genital glands. The spaces between the organs are filled with connective tissue - parenchyma.
Digestive system has three sections: anterior (oral cavity, pharynx, esophagus), middle (stomach, middle intestine) and posterior (hind intestine, anus). There is a liver, salivary glands (many). The horny jaws are located in the oral cavity. There is a tongue in the throat ( grater, or radula), covered with teeth. The hindgut opens into the mantle cavity. Molluscs feed on plant and animal food. They actively swallow it or passively filter the water.
Circulatory system open. The heart is located in the pericardial sac and has 1 ventricle and 1–2 or 4 atria. Blood enters the vessels, and then into the gaps between the organs - lacunae... It washes the organs, then collects in the vessels that go to the respiratory organs, and from there to the heart. Blood is often colorless, sometimes it contains a substance similar in structure to hemoglobin.
Respiratory system. In aquatic forms - skin gills(folds of the mantle), in terrestrial forms - lung(robe pocket) with breathing hole.
Excretory organs - kidneys(modified metanephridia). They open at one end into the pericardial sac, the other into the mantle cavity.
Nervous system diffuse-nodular type... It consists of nerve nodes located in different parts of the body and interconnected by nerve trunks.
Sense organs represented by the organs of sight (eyes), touch, balance and chemical sense.
Reproduction and development. There are dioecious and hermaphrodites. Sexual reproduction. The sex glands (testes and ovaries) are paired. Insemination, external or internal. Development is direct (in cephalopods and some gastropods) or with metamorphosis (in bivalves and some gastropods). Larva - sailboat(in gastropods) or glochidia(in bivalves).
Mollusks move with the help of their legs (wave-like muscle contractions) or reactively (pushing out water when the shell is abruptly closed or through a funnel from the mantle cavity).
Origin and aromorphosis. Molluscs evolved from annelids. The following aromorphoses led to the emergence of the type: division of the body into sections; the appearance of the heart, kidney, liver.

Class Gastropods

Representatives: grape snails, pond snails, coils, slugs, rapana, etc. The habitat is water and ground-air. They live in fresh water bodies, seas, damp land areas.
A characteristic feature is the asymmetry of the structure, due to the reduction of organs on the right and the predominant development of organs on the left side. The shell is whole, spirally twisted or reduced (in slugs). The mantle partially covers the body, forming the so-called lung with a breathing hole. In the mouth there is a grater formed by horny teeth. There are one or two pairs of tentacles on the head. Eyes are located at their base or at the ends of the first pair. There are both herbivorous snails (they eat by scraping off algae or tissues of higher plants - a pond snail, a coil, a grape snail), and predatory forms (rapans eat mussels, oysters).
Meaning. People use grape snails for food. Many gastropods are pests of agricultural plants (slugs, grape snails, etc.). The small pond snail serves as an intermediate host for the liver fluke. Predatory snails (rapans) harm oyster and mussel settlements.

Class Bivalves

Class Cephalopods

Representatives: octopus, squid, cuttlefish, etc. Highly organized molluscs. They live mainly in warm seas and oceans. All predators. The reactive method of movement is characteristic.
The body consists of a head and torso. Leg converted to tentacles (arms) surrounding the mouth opening. The shell is internal, often reduced or absent. There is cartilaginous "skull" and two thick horny jaw (beak), by which food is captured and crushed. Cephalopods have two pairs of salivary glands, the discharge of one of them can be poisonous. The circulatory system is usually closed. The heart has 1 ventricle and 4 atria. A duct opens into the hind gut ink gland... The brain has a complex structure. A pair of large eyes is very similar in structure to the eyes of mammals. Cephalopods are dioecious, reproduce, as a rule, once in a lifetime. Direct development.
Meaning. Fishing object (cuttlefish, squid, octopus). Source of pharmaceutical raw materials. From the secret ink bag cuttlefish and squid receive Chinese ink and sepia watercolor.

Arthropod type

general characteristics

Body covers presented cuticle and hypodermis... The musculocutaneous sac, characteristic of the previous groups, is reduced, which is associated with the presence of a dense outer cover. The cuticle is formed by chitin. Chitin can be impregnated with lime salts (shells of higher crustaceans) or proteins (insects). The chitinous cover performs a protective function - it protects against drying out and mechanical stress... Thanks to him, arthropods were the first animals to inhabit the land. In addition, the chitinous cover is the outer skeleton - bundles of striated muscles are attached to its inner surface. The emergence of this type of musculature has provided an increase in mobility. The chitinous cover is inextensible, therefore the growth of arthropods is accompanied by molt.
Organs of movement. In primitive arthropods, each body segment has a pair articulated limbs... The limbs are movably connected to the body by joints. In the process of evolution, some of the limbs were lost, others specialized to perform a certain function and were transformed into sensory organs, mouth organs, walking and swimming limbs, gills, spider web warts, etc.
Body cavity mixed - mixocel... It is formed when the sections of the primary and secondary cavities merge.
Digestive system has three sections - anterior (mouth, pharynx, esophagus, sometimes a goiter), middle (stomach, middle intestine) and posterior (hind intestine and anus). The anterior and posterior regions have a cuticular lining. There are liver and salivary glands. Complex appears oral apparatus from the modified forelimbs. It is specialized for a certain type of food (gnawing, licking, sucking, piercing-sucking, etc.).
Circulatory system open. There is heart located on the dorsal side of the body. Hemolymph circulates through the vessels. It is a colorless liquid, which has a double nature: partly corresponds to blood, partly to cavity fluid. From vessels hemolymph pours into the body cavity and washes the internal organs. Then it enters the blood vessels and the heart again.
Respiratory system. Primary aquatic arthropods have gills, for terrestrial - pulmonary sacs and trachea(chitinous tubes that permeate the whole body).
Excretory system represented by modified metanephridia ( green and coxal glands), fatty body (accumulation kidney) or malpighian vessels (intestinal outgrowths). Crustaceans have green glands, arachnids have malpighian vessels and coxal glands, and insects have malpighian vessels and fatty body.
Nervous system consists of the supraopharyngeal and subopharyngeal nerve nodes (ganglia), connected by nerve cords in the periopharyngeal ring, and the abdominal nerve chain.
Senses: sight, taste, touch, smell, hearing and balance.
Reproduction and development. As a rule, dioecious. Sexual dimorphism is well expressed. The female has the ovaries and oviducts, the male has the testis, the vas deferens and the ejaculatory duct. Reproduction is only sexual, parthenogenesis and viviparity occur. Development can be direct, with complete or incomplete metamorphosis. Growth is possible only with periodic molting - the shedding of the old cuticle and the formation of a new one.
Origin and aromorphosis. Arthropods are descended from ancient marine annelids. The following aromorphoses led to the emergence of the type: the emergence of the external skeleton, articulated limbs, striated muscles.

Crustacean class

Extremities. The cephalothorax and abdomen consist of unequal segments, each of which corresponds to a pair of articulated limbs, specialized to perform a specific function. The crayfish has the following limbs: segments cephalothorax carry 13 pairs of limbs: antennae(organs of smell), antennas(organs of touch), upper jaws and 2 pairs of lower jaws (grinding food); 3 pairs of legs (feeding food into the mouth) and 5 pairs of walking legs (movement), 1st pair of walking legs converted into pincers (defense and attack); on the abdomen 6 pairs of limbs: 5 pairs of swimming legs (in the male the 1st and 2nd pairs are the copulatory organ, in the female the swimming legs hold eggs and cubs), the limbs of the 6th pair together with the 7th abdominal segment form the caudal fin.