Single-celled organisms are microscopically small, and this imposes restrictions on the possibility of complication and the appearance of various organs for a more efficient development of the habitat. The easiest way is to increase the size of the cell, but this way turns out to be a dead end. The size of the cells is limited by the ratio of surface and volume. Let us assume that the cube-cell has a face length of 1 cm. Let's double the size and compare the ratio of the surface areas and volumes of the large and small cells. Formation of multicellular organisms

Cube area: 1 x 1 x 6 \u003d 6 cm 2 Volume: 1 3 \u003d 1 cm 3 Ratio \u003d 6: 1 If the face of the cube doubles, then the area of \u200b\u200bthe cube: 2 x 2 x 6 \u003d 24 cm 2 Volume: 2 3 \u003d 8 cm 3 Ratio \u003d 3: 1 The surface has increased by 4 times, and the volume - by 8 times, which means that for each unit of surface there will now already be two units of volume. Hence it follows that with an increase in size: the cell starts to starve, the surface will not provide the entire volume with nutrients, especially by diffusion; gas exchange is difficult; elimination of waste products is difficult; heat transfer is difficult. Formation of multicellular organisms

This means that the size of the cell is limited, and the increase in size is associated with the formation of multicellular organisms. How did multicellular organisms arise? E. Haeckel suggested that the volvox-like ancient organism, similar to blastula, has undergone a simple change. Its single-layer wall began to protrude inward, the mouth opening and the primary intestinal cavity, the outer layer of ectoderm cells, and the inner endoderm were formed. This process is called intussusception, and the resulting organism gastrula (from the Latin "gaster" stomach), which has a primary digestive system... This theory is called the theory of gastrea. Formation of multicellular organisms

One of our greatest zoologists II Mechnikov disagreed with E. Haeckel. He believed that intussusception is a secondary process. II Mechnikov, studying the ontogeny of lower multicellular organisms, found that in many of them the second layer of endoderm cells is formed not by invagination, but as a result of migration of amoeboid cells into the colony and, multiplying there, they form a parenchyma. These cells are capable of amoeboid movement and phagocytosis. To capture large food particles, an opening appears, to which food particles are adjusted using flagella. Food enters the colony and is surrounded by amoeboid cells, which form the second germ layer of the endoderm. Formation of multicellular organisms

The rest of the amoeboid cells became parenchyma, they provide transmission nutrients all cells of the body. Thus, the cells equipped with flagella took over the function of movement, and those that went into the primary cavity took over the function of reproduction and nutrition. The theory of the origin of multicellular animals according to I.I. Mechnikov is called the theory of phagocytella. Both points of view have their supporters, it is possible that both scientists are right and multicellular organisms were formed different ways... Formation of multicellular organisms

Since 1883, animals have been known that belong to the most primitive multicellular animals and constitute a separate type of Placozoa Trichoplax (Trichoplax). The size of these animals is no more than 4 mm, trichoplax is a flat plate slowly crawling over the substrate in seawater. The most surprising thing is that it does not have endoderm, it is, as it were, a blastula flattened over the surface of the substrate. The lower layer is formed by cells that have flagella. It turned out that surface cells, capturing food particles, migrate to the parenchyma, where food is digested. It can be considered that in Trichoplax, the endoderm is in the stage of formation. The discovery of trichoplax strongly supported the theory of I.I.Mechnikov. Type Lamellar (Placozoa).

Apart from lamellar animals, sponges are the simplest multicellular animals. These sedentary animals, mainly marine ones, do not have organs and tissues, although their various cells perform different functions. Nervous system is absent, internal cavities are lined with choanocytes with special flagellate collar cells. Sponge Type (Spongia, or Porifera)

Almost all sponges have a complex mineral or organic skeleton. The simplest sponges are in the form of a bag, which is attached to the substrate with its base, and the opening with the mouth) is directed upward. The walls of the sac are composed of two layers of cells. It is believed that the outer layer of the ectoderm is the inner endoderm (in fact, just the opposite). Sponge Type (Spongia, or Porifera)

A structureless mass of mesogley is located between the layers of cells, in which numerous cells are located, including the needles forming spicules internal skeleton... The entire body of the sponge is permeated with thin channels leading to the central, paragastric cavity. The continuous operation of the flagella creates a flow of water through the channels into the cavity and through the mouth (osculum) outward. Sponge Type (Spongia, or Porifera)

The sponge feeds on those food particles that water brings. This is the simplest type of structure of the Ascon sponges. But in most sponges, a thickening of the mesoglea occurs and flagellate cells line the invaginations, cavities. This type of structure is called sicon, and when these cavities completely go inside the mesoglea and are connected by channels with the paragastric cavity, leukon. Sponge Type (Spongia, or Porifera)

Sponges, moreover, usually form colonies with many openings on the surface: in the form of crusts, lumps of lumps, bushes. In addition to asexual reproduction of budding, sponges also reproduce sexually. The way the larva develops is remarkable. Sponge Type (Spongia, or Porifera)

From the egg, a blastula develops, consisting of one layer of cells, and at one pole the cells are small and with flagella, at the other large ones without flagella. First, large cells invade inward, then protrude and the larva floats freely, then flagellate cells invaginate again, which become the inner layer. Sponge Type (Spongia, or Porifera)

The larva settles and turns into a young sponge (4). The peculiarities of the embryonic development of sponges give reason to scientists to believe that in them the primary ectoderm (small flagellated cells) replaces the endoderm. There is a perversion of the embryonic layers. On this basis, zoologists give the name to the sponges animals turned inside out (Enantiozoa). Sponge Type (Spongia, or Porifera)

It is interesting that the larva of most parenchymal sponges, in structure, almost completely corresponds to the hypothetical phagocytella of II Mechnikov. It has a surface layer of flagellate cells, under which the cells of the inner loose layer are located. It can be assumed that the phagocytella switched to a sedentary lifestyle and in this way gave rise to the Sponge type. Sponge Type (Spongia, or Porifera)

Another feature is the amazing ability of the sponges to regenerate. Even when rubbed through a sieve and turned into a slurry, consisting of cells or their groups, they are capable of restoring the body. If you rub two sponges through a sieve and mix these masses, then the cells of different animals will gather in two different sponges. In nature, sponges are essential as biofilters. Settling in reservoirs with significant organic pollution, they participate in their biological treatment. Sponge Type (Spongia, or Porifera)

The practical value of sponges is not great. In some southern countries, the fishery of toilet sponges with a horny skeleton is developed; freshwater sponge is used in folk medicine... The sponges have practically no enemies, except for some sea stars. Others are frightened off not only by the spiky skeleton, but also by the sharp, specific smell of the substances they release. These substances are toxic to many animals. But on the other hand, sponges in cavities and voids have many lodgers and parasites of small crustaceans, worms, molluscs living under their protection. Sponge Type (Spongia, or Porifera) Badiaga Cup of Neptune

Body Placozoa composed of the outer epithelial layer of flagellar cells and the inner mass of amoeba-like cells - parenchyma.

Until now, only two representatives of this type are known: Trichoplax adhaerens and Trichoplax reptans, both described at the end of the last century, but until recently were mistaken for aberrant coelenterate larvae. Only in 1971 was it possible to observe the sexual reproduction of Trichoplax and prove that it is a normal adult organism.

Trichoplax - a sea creature crawling on the surface of algae. Its body is in the form of a very thin grayish plate, no more than 4 mm in diameter. The animal slowly glides on its lower surface adjacent to the substrate, and at the same time changes shape. The direction of movement is also easily changed; the body does not have constant front and rear ends and a certain symmetry. The creeping Trichoplax resembles a giant amoeba (Fig. 1A).

Structure and physiology

The lower cell layer adjacent to the substrate, conventionally called the “abdominal” layer, consists of tall cells, each carrying one cord (Fig. 1, B). The upper, or "dorsal", cell layer has the characteristics of the so-called submerged epithelium. Each of its cells consists of a cytoplasmic plate with one cord lying on the surface and a cell body with a nucleus immersed in the parenchyma. Some of these cells contain a fairly large fatty (lipid) vacuole. It is characteristic that the integumentary layer of cells is not delimited by anything from the parenchyma (the main, or basal, membrane is absent).

The entire internal space of the animal is filled with a mass of very diverse amoeboid cells that can move through pseudopodia. Many cells of the abdominal epithelium, apparently, lose their bundle, sink into the body and turn into amoeba-like elements. The same happens with some cells of the dorsal epithelium, although to a lesser extent. Among the cellular elements of the parenchyma, large and spindle-shaped cells are especially distinguished, which stretch from the ventral side of the body to the dorsal and have a contractile function. Trichoplax can cover accumulations of food particles (for example, flagellates Cryptomonas), pour out on them the digestive secretions of the cells of the abdominal epithelium and it is possible to then absorb the products of external digestion with their surface. At the same time, the presence of parenchyma of digestive vacuoles in some amoebocytes suggests that nutrition is also carried out through phagocytosis. The mechanism of "amoeboid" movement in Trichoplax, which is completely devoid of muscular elements remains mysterious. It can only be assumed that the spindle-shaped cells of the parenchyma with their mitochondrial complex are capable of contracting and that this is directly related to the movement of the animal. However, this is hardly the only explanation for all changes in body shape.

Figure: 1. Organization Trichoplax.

A - Trichoptax adhaerens. Changes in the body shape of one individual, sketched every 10 minutes (according to Schulze);

B - cross section through Trichoplax sp... (according to Ivanov);

IN - crushing eggs Trichoplax adhaerens (no Grell):

1 - dorsal epithelium;

2 - mesenchymal cells of the middle layer,

3 - abdominal epithelium,

4-digestive vacuole in the mesenchymal cell (according to Grell)

Reproduction and development

Asexual reproduction was observed in the last century. Trichoplax by dividing the body in two. Budding has also been recently described. It occurs on the dorsal side of the body and leads to the separation of small vagrants, able to swim quickly with the help of tourniquets and serving to disperse the species.

During sexual reproduction, gonocytes appear in the trichoplax parenchyma, first associated with the abdominal layer of flagellate cells and then turning into eggs rich in yolk. No sperm has been found. However, judging by the primary shell that appears around each mature egg, fertilization occurs, after which the egg undergoes complete uniform cleavage, which in some respects resembles a very primitive spiral cleavage (Fig. 1, C).

Phylogeny type Placozoa

By organization level Trichoplax corresponds to the parenchymula, the characteristic larva of sponges and coelenterates, which probably recapitulates the main features of the phagocytella, the supposed common ancestor of all multicellular animals. Therefore, we can think that Placozoa are the closest descendants of the phagocytella, which have passed from the original free-swimming lifestyle to crawling on the surface of algae. At the same time, their body lost its primary anteroposterior polarity and turned into a thin plate. Opening Placozoa - a new confirmation of the correctness of the theory of I.P. Mechnikov about the origin of multicellular animals.

Type Lamellar / Dogel V.A.Zoology of invertebrates - 7th ed., M., 1981 p. 98-100

SUPER SECTION PHAGOCYTELLOZOA

The most primitive multicellular animals that have retained the basic structural features of the primitive Metazoa. These include one type.

TYPE PLATE ANIMALS (PLACOZOA)

The body of Placozoa is composed of the outer epithelial layer of flagellate cells and the inner mass of amoeba-like cells - the parenchyma.

Until now, only two representatives of this type are known: Trichoplax adhaerens and Trichoplax reptans, both were described at the end of the last century, but until recently were mistaken for aberrant coelenterate larvae. Only in 1971 was it possible to observe the sexual reproduction of Trichoplax and prove that it is a normal adult organism.

Trichoplax - a sea creature crawling on the surface of algae. Its body is in the form of a very thin grayish plate, no more than 4 mm in diameter. The animal slowly glides on its lower surface adjacent to the substrate, and at the same time changes shape. The direction of movement is also easily changed; the body does not have constant front and rear ends and a certain symmetry. Creeping Trichoplax resembles a giant amoeba.

Structure and physiology. The lower cell layer adjacent to the substrate, conventionally called the "abdominal", consists of tall cells, each carrying one bundle. The upper, or "dorsal", cell layer has the characteristics of the so-called submerged epithelium. Each of its cells consists of a cytoplasmic plate with one cord lying on the surface and a cell body with a nucleus immersed in the parenchyma. Some of these cells contain a rather large fatty (lipid) vacuole. It is characteristic that the integumentary layer of cells is not delimited by anything from the parenchyma (the main, or basal, membrane is absent).

The entire internal space of the animal is filled with a mass of very diverse amoeboid cells that can move through pseudopodia. Many cells of the abdominal epithelium, apparently, lose their bundle, sink into the body and turn into amoeba-like elements. The same happens with some cells of the dorsal epithelium, although to a lesser extent.

Among the cellular elements of the parenchyma, large and fusiform cells are especially prominent, which stretch from the ventral side of the body to the dorsal and have a contractile function.

Trichoplax can cover accumulations of food particles (for example, flagellates Cryptomonas), pour out on them the digestive secretions of the cells of the abdominal epithelium and it is possible to then absorb the products of external digestion with their surface. At the same time, the presence of parenchyma of digestive vacuoles in some amoebocytes indicates that nutrition is also carried out through phagocytosis.

The mechanism of "amoeboid" movement in Trichoplax, which is completely devoid of muscular elements remains mysterious. It can only be assumed that the spindle-shaped cells of the parenchyma with their mitochondrial complex are capable of contracting and that this is directly related to the movement of the animal. However, this is hardly the only explanation for all changes in body shape.

Reproduction and development.Asexual reproduction was observed in the last century Trichoplax by dividing the body in two. Budding has also been recently described. It occurs on the dorsal side of the body and leads to the separation of small vagrants, which are able to swim quickly with the help of bundles and serve to disperse the species.

During sexual reproduction, gonocytes appear in the trichoplax parenchyma, first associated with the abdominal layer of flagellate cells and then turning into eggs rich in yolk. No sperm has been found. However, judging by the primary shell that appears around each mature egg, fertilization occurs, after which the egg undergoes complete uniform cleavage, which in some respects resembles a very primitive spiral cleavage.

Phylogeny typePlacozoa

By organization level Trichoplax corresponds to the parenchymula - the characteristic larva of sponges and coelenterates, which probably recapitulates the main features of the phagocytella, the supposed common ancestor of all multicellular animals. Therefore, we can think that Placozoa are the closest descendants of the phagocytella, which have passed from the original free-swimming lifestyle to crawling on the surface of algae. At the same time, their body lost its primary anteroposterior polarity and turned into a thin plate.

The discovery of Placozoa is a new confirmation of the correctness of the theory of II Mechnikov about the origin of multicellular animals.

TYPE PLATE (PLACOZOA)
The lamellar type includes only two species of one genus of marine animals - Trichoplax. Trichoplaxis are lamellar marine animals about 3 mm in size. They live on algae. Outwardly, they resemble a large amoeba, since they do not have a constant body shape and when they move, their contours change.
However, they move with the help of flagellar cells that cover the body. The flagellar cells of the ventral side are narrow and tall, alternating with glandular cells, while the flagellar cells of the dorsal side are flattened. Fusiform and amoeboid cells are located inside the body.
Trichoplaxis can be fed by external digestion and by phagocytosis. In the first case, enzymes are released from the glandular cells of the ventral side of the trichoplax body onto the film of bacteria covering the substrate. In this case, food is digested outside the body of the animal and then absorbed by the same glandular cells. But the main way of feeding Trichoplax is phagocytosis.

Food particles are driven by flagellate cells of the dorsal side to the surface of the body, and then swallowed by fusiform cells, which are able to push pseudopodia through the spaces between integumentary cells. Cells overloaded with digestive vacuoles - phagocytes sink into the body and become short, amoeba-like.
Trichoplaxis reproduce asexually and sexually. With asexual reproduction, the body of Trichoplax is lacing and divided into two parts. During sexual reproduction, eggs and spermatozoa are formed in the thickness of the body of animals. However, the development of Trichoplaxis has not been studied enough.
The structural features and physiology of Trichoplaxis indicate that they are the most primitive modern multicellular organisms, similar to their hypothetical ancestor - phagocytella.

SUBMARINE MULTICELLE (METAZOA)

Supersection I. Phagocytellozoa (Phagocytellozoa)
The supersection of phagocyteliforms includes the most primitive multicellular animals. These include only one type - Lamellar animals (Placozoa). Representatives of lamellar cells have been known to science since the middle of the last century, but were described as an independent type of animals only in 1971. Phagocytelloids have two main types of cells: external flagellates and internal phagocytic cells, in which food is digested. They have no mouth, no digestive cavity, no tissues, organs. By their organization, they are similar to the hypothetical ancestor of multicellular organisms - phagocytella (according to Mechnikov), which gave rise to the name of the supersection.

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Presentation on the topic: Plate type, Sponge type

Slide No. 1

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Slide No. 2

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Formation of multicellular organisms Single-celled organisms are microscopically small, and this imposes restrictions on the possibility of complication and the appearance of various organs for more efficient development of the habitat. The easiest way is to increase the size of the cell, but this path turns out to be a dead end - the size of the cells is limited by the ratio of the surface and volume. Let's say that the cube-cell has a face length of 1 cm. Let's double the size and compare the ratio of the surface areas and volumes of the large and small cells.

Slide No. 3

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Formation of multicellular organisms Cube area: 1 x 1 x 6 \u003d 6 cm2 Volume: 13 \u003d 1 cm3 Ratio \u003d 6: 1 If the face of a cube doubles, then the area of \u200b\u200bthe cube: 2 x 2 x 6 \u003d 24 cm2 Volume: 23 \u003d 8 cm3 Ratio \u003d 3: 1 Surface increased by 4 times, and the volume - by 8 times, which means that for each unit of surface there will already be two units of volume. It follows that with an increase in size: the cell begins to starve, the surface will not provide the entire volume with nutrients, especially by diffusion; gas exchange is difficult; elimination of waste products is difficult; heat transfer is difficult.

Slide No. 4

Slide Description:

Formation of multicellular organisms Hence, the size of the cell is limited, and the increase in size is associated with the formation of multicellular organisms. How did multicellular organisms arise? E. Haeckel suggested that the volvox-like ancient organism, similar to blastula, has undergone a simple change. Its single-layer wall began to protrude inward, the mouth opening and the primary intestinal cavity were formed, the outer layer of cells was the ectoderm, and the inner layer was the endoderm. This process is called invagination, and the resulting organism is called gastrula (from the Latin "gaster" - stomach), which has a primary digestive system. This theory is called the theory of gastrea.

Slide No. 5

Slide Description:

Formation of multicellular organisms One of our greatest zoologists I.I. Mechnikov did not agree with E. Haeckel. He believed that intussusception is a secondary process. I. I. Mechnikov, studying the ontogeny of lower multicellular organisms, found that in many of them the second layer of cells - the endoderm - is formed not by invagination, but as a result of migration of amoeboid cells into the colony and, multiplying there, they form a parenchyma. These cells are capable of amoeboid movement and phagocytosis. To capture large food particles, an opening appears, to which food particles are adjusted using flagella. Food enters the colony and is surrounded by amoeboid cells, which form the second germ layer, the endoderm.

Slide No. 6

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Formation of multicellular organisms The remaining amoeboid cells have become parenchyma, they provide the transfer of nutrients to all cells of the body. Thus, the cells equipped with flagella took over the function of movement, and those that went into the primary cavity took over the function of reproduction and nutrition. According to I.I. Mechnikov, the theory of the origin of multicellular animals is called the theory of phagocytella. Both points of view have their supporters, it is possible that both scientists are right and multicellular organisms were formed in different ways.

Slide No. 7

Slide Description:

Type Lamellar (Placozoa). Since 1883, animals have been known that belong to the most primitive multicellular animals and constitute a separate type Lamellar (Placozoa) - Trichoplax (Trichoplax). The size of these animals is no more than 4 mm, trichoplax is a flat plate slowly crawling over the substrate in sea water. The most surprising thing is that it does not have endoderm, it is like a blastula flattened over the surface of the substrate. The lower layer is formed by cells that have flagella. It turned out that surface cells, capturing food particles, migrate to the parenchyma, where food is digested. It can be considered that in Trichoplax, the endoderm is in the stage of formation. The discovery of trichoplax strongly supported the theory of I.I.Mechnikov.

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Type Sponges (Spongia, or Porifera) Apart from the lamellar animals, sponges are the simplest multicellular animals. These sedentary animals, mainly marine ones, do not have organs and tissues, although their various cells perform different functions. The nervous system is absent, the internal cavities are lined with choanocytes - special flagellate collar cells.

Slide No. 9

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Type Sponges (Spongia, or Porifera) Almost all sponges have a complex mineral or organic skeleton. The simplest sponges are in the form of a bag, which is attached to the substrate with its base, and the opening with the mouth) is directed upward. The walls of the sac are composed of two layers of cells. It is believed that the outer layer is the ectoderm, the inner one is the endoderm (in fact, just the opposite).

Slide No. 10

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Sponge type (Spongia, or Porifera) Between the layers of cells there is a structureless mass - mesoglea, in which numerous cells are located, including those forming spicules - the needles of the internal skeleton. The entire body of the sponge is permeated with thin channels leading to the central, paragastric cavity. The continuous operation of the flagella creates a flow of water through the channels into the cavity and through the mouth (osculum) outward.

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Type of Sponge (Spongia, or Porifera) The sponge feeds on the food particles that the water brings. This is the simplest type of sponge structure - ascon. But in most sponges, a thickening of the mesogley occurs and flagellate cells line the invaginations, cavities. This type of structure is called sicon, and when these cavities completely go inside the mesoglea and are connected by channels with the paragastric cavity - leukon.

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Type Sponges (Spongia, or Porifera) Sponges also usually form colonies with many openings on the surface: in the form of crusts, lumps of lumps, bushes. In addition to asexual reproduction - budding, sponges also reproduce sexually. The way the larva develops is remarkable.

Slide No. 14

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Sponge type (Spongia, or Porifera) A blastula, consisting of one layer of cells, develops from the egg cell, and at one pole the cells are small and with flagella, at the other - large without flagella. First, large cells invade inward, then protrude and the larva floats freely, then flagellate cells invaginate again, which become the inner layer.

Slide Description:

Type of Sponges (Spongia, or Porifera) It is interesting that the larva of most sponges is a parenchymula, in structure almost completely corresponding to the hypothetical phagocytella of II Mechnikov. It has a surface layer of flagellate cells, under which the cells of the inner loose layer are located. It can be assumed that the phagocytella switched to a sedentary lifestyle and in this way gave rise to the Sponge type.

Slide No. 17

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Type of Sponge (Spongia, or Porifera) Another feature is the amazing ability of the sponges to regenerate. Even when rubbed through a sieve and turned into a slurry, consisting of cells or their groups, they are capable of restoring the body. If you rub two sponges through a sieve and mix these masses, then the cells of different animals will gather in two different sponges. In nature, sponges are essential as biofilters. Settling in water bodies with significant organic pollution, they participate in their biological treatment.

Slide No. 18

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Type of Sponge (Spongia, or Porifera) The practical value of sponges is not great. In some southern countries, the fishery of toilet sponges with a horny skeleton is developed; a freshwater sponge is used in folk medicine. Sponges have practically no enemies, except for some starfish. Others are frightened off not only by the spiky skeleton, but also by the sharp, specific smell of the substances they release. These substances are toxic to many animals. But on the other hand, sponges in cavities and voids have many lodgers and parasites - small crustaceans, worms, mollusks living under their protection.