Evidence of the relationship of multicellular and unicellular animals. Origin of multicellular organisms. The external structure of the body

The simplest are the oldest inhabitants of the Earth. The evolution of animals did not stop at the unicellular stage of organization: as a result of aromorphoses, numerous multicellular forms appeared. However, due to idioadaptations, the protozoa are well and diversely adapted to the conditions of their habitat and, apparently, without noticeable morphophysiological changes, exist for millions of years, being in a state of biological progress.

In science, there is no single point of view about which of the classes of the simplest to recognize as the oldest. There are solid arguments in favor of recognizing Sarcode as the oldest. They do not have a permanent body shape, thus resembling the first primitive living beings, they have a low degree of differentiation.

Later, in some amoeba-like protozoa, pseudopodia turned into flagella. Primary flagellates retained amoeba-like features for a long time, as evidenced by the fact that even modern mastigamebes are capable of forming pseudopodia. In the process of evolution, the primary heterotrophic flagellates gave rise to a branch of autotrophic flagellates, from which, possibly, plant organisms originated.

However, until now, many zoologists tend to consider the primary flagellates (Protophlagellata) as the ancestors of all classes of protozoa (and, consequently, of the entire organic world). Anyway, genetic connections between sarcodal and flagella are undoubted.

Ciliates are similar to flagellates in a number of ways ( permanent form bodies, similarity in the structure of flagella and cilia). Primary flagellates should be recognized, apparently, also the ancestors of the ciliates. However, a number of new data force us to reconsider the origin of protozoa in a new way.

There is no doubt that eukaryotes evolved from prokaryotes. Back in 1924, the Soviet botanist B. M. Kozo-Polyansky (1890-1957) suggested the symbiotic origin of cells, in particular their motor organelles. By interesting hypothesis American biologist L. Sagan-Margulis (1967), once in the Late Precambrian, a spirochete-like bacterium entered into symbiosis with an amoeboid eukaryotic creature. As a result, not only the mobility of the symbiotic organism increased, but the basal granule of the bacterium took on a new function - centrioles, which was a prerequisite for mitotic division. The occurrence of mitosis new era in the history of life on earth. The descendants of some of the eukaryotic organisms acquired chlorophyll (also possibly as a result of symbiosis with blue-green algae) and gave rise to green plants. Others retained a heterotrophic type of nutrition. Fungi and animals, including modern protozoa, originated from them. At the same time, in some the flagellum was preserved, while in others it was lost in the process of evolution. This hypothesis makes the dispute about who is considered the most ancient - sarcode or flagella unpromising.

The origin of multicellularity is associated with a major aromorphosis in the evolution of the organic world, expressed in the formation of germ layers. The most probable origin of multicellular organisms is from a colony of flagellates of the Volvox type. However, the most complex colony of protozoa always remains single-layered, while the most primitive multicellular colonies remain double-layered. The main difficulty lies in explaining the transition from a single layer colony unicellular organisms to a bilayer multicellular organism. Such organisms are sponges and coelenterates - the most primitive multicellular organisms.

The first attempt to resolve this issue belongs to E. Haeckel (1874). In constructing his hypothesis, he proceeded from the embryological studies carried out by that time by A. O. Kovalevsky and other zoologists, mainly on the lancelet and a number of vertebrates. Based on the biogenetic law, Haeckel believed that each stage of ontogeny repeats some stage passed by the ancestors of a given species during phylogenetic development. According to him, the zygote stage corresponds to unicellular ancestors, the blastula stage - a spherical flagellate colony. Further, according to this hypothesis, an invagination (invagination) of one of the sides of a spherical colony occurred (as during gastrulation in a lancelet) and a hypothetical two-layer organism was formed, called by Haeckel gastreya (since it looks like a gastrula and occurred like it), and Haeckel's hypothesis was called the theory of gastraea. Despite the mechanistic reasoning of Haeckel, who identified the stages of ontogenesis with the stages of evolution of the organic world without taking into account their physiological and ecological characteristics, the theory of gastrea played an important role in the history of science, as it contributed to the approval of monophyletic ideas on the origin of multicellular organisms.

In 1886, I. I. Mechnikov criticized it and substantiated his theory of the origin of multicellular organisms. The data of comparative embryology speak in favor of the theory of I. I. Mechnikov, comparative anatomy and comparative physiology, which is why it has gained wide popularity among zoologists.

I. I. Mechnikov studied the embryonic development of lower multicellular organisms and showed that their gastrula never forms by invagination: in the process of gastrulation, part of the surface cells of the blastula immigrate into the cavity, resulting in the formation of two layers - the outer (ectoderm) and the inner (endoderm) .

According to II Mechnikov, in a hypothetical ancestor of multicellular organisms - a spherical flagellate colony - cells that captured food particles temporarily lost their flagella and moved inside the colony. Then they could return to the surface again and restore the flagellum. Gradually, in a spherical colony, there was a division of functions between the members of the colony. Successful capture of food requires active movement, which led to the polarization of the body. The anterior cells acquired a specialization in relation to movement (formed a kinoblast), while the posterior ones, where the turbulence of water currents created conditions more favorable for the capture of particles, specialized in relation to nutrition (formed a phagocytoblast). The resulting difficulty in the transfer of food from the posterior cells to the anterior ones led to the immigration of phagocytoblasts into the body cavity. To capture larger prey, a special opening (mouth) arose. This hypothetical organism is very similar to the larvae of many sponges and coelenterates, in connection with which I. I. Mechnikov originally called it parenchymella. Later, I. I. Mechnikov considered this name unsuccessful and, given that the inner layer of a hypothetical organism is formed from cells that perform a phagocytic function, he called it phagocytella. The hypothesis of I. I. Mechnikov was called the theory of phagocytella.

Hypotheses of the origin of multicellular animals

The gastrea hypothesis E. Haeckel (1874).

A transitional form between unicellular and multicellular animals is a single-layer spherical flagellate colony - a “blastea”, resembling a blastula.

In the process of evolution from the "blastea" by invagination of the wall of the colony, the first multicellular - "gastrea" originate, from which the intestinal animals and other groups of multicellular organisms originate. E. Haeckel considered the presence of blastula and gastrula stages in the early stages of ontogeny of modern multicellular organisms as proof of the correctness of his hypothesis.

The "plakula" hypothesis O. Byuchli (1884) is a variant of Haeckel's gastraea hypothesis. Unlike E. Haeckel, this scientist accepts a lamellar single-layer colony as a transitional form between unicellular and multicellular animals.

The 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. While studying the ontogeny of primitive multicellular sponges, he discovered that the gastrula in sponges is formed not by invagination of the blastula, but by immigration of some cells of the outer layer into the cavity of the embryo.

It was these two discoveries that formed the basis for this hypothesis. The prototype, or living model of the hypothetical ancestor of multicellular organisms - "phagocytella" - I.I. Mechnikov considered the larva of sponges - parenchymula.

The main hypotheses are given in rice. 2.1.

Rice. 2.1. Scheme of the origin of multicellular (colonial):

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

2.2. Sponge type. Genus Badyaga (Spongilla)

Sponge type characteristic

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 - 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 color to sponges; amebocytes- amoeboid cells that carry nutrients from choanocytes to all cells of the body.

TO ollencites- star-shaped cells that perform connective tissue and support functions; sex cells from which gametes are formed; archeocytes- cells capable of transforming into all other types of cells; scleroblasts- cells that form skeletal 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 the flow of water, 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 their bodies, 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 a sponge with a stream of water.

Nervous system

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

Reproduction (reproductive organ system)

Sponges are breeding asexual And sexual way. asexual reproduction occurs by external budding and leads to the formation of colonies.

Sponge badyaga is dioecious animals, but there are no external sex differences. Fertilization occurs in the female colony, where spermatozoa are brought with a stream of water.

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

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

Those. the germ layers are interchanged, which is why 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 has passed to sedentary image life and in this way gave rise to the Sponge type.

Regeneration well expressed

Value in nature

positive

Biofilters are involved in the biological purification of water. They constitute an important link in the ecosystems of rivers and play a significant role in their hydrobiological regime. They participate in trophic 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 shelter 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. Therapeutic action Sponge preparations are based on mechanical irritation of the skin by spicules, irritation is accompanied by blood flow to the diseased area of ​​the body.

Rice. 2.2. Sponge structure

2.3. Type Intestinal (Coelenterata, Radiata)

General characteristics of the type

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

Ecology

These animals lead an aquatic lifestyle and are common in all seas and freshwater reservoirs. These are lower, predominantly marine, multicellular animals attached to the substrate or floating in the water column.

life forms

Two life forms: sessile saccular polyp and free-floating discoid jellyfish. Both forms may 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 the polyp.

Respiration (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 (cavitary and intracellular digestion). Undigested food debris is expelled through the mouth.

Excretion (excretory system)

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

Nervous system

Nervous system diffuse type

Origin

From colonial protozoa - flagellates.

Class 1. Hydroids

View Hydra freshwater (typical representative)

Predatory animal. With the stinging threads of its tentacles, it strikes small aquatic animals, paralyzing and swallowing them.

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

ectoderm(outer layer of the body) consists of several types of cells:

- epithelial-muscular cells (movement)

– intermediate cells (regeneration)

- stinging cells (defense and attack)

- sex cells (sexual reproduction)

- nerve cells (forming diffuse nervous system)

Endoderm( inner layer):

Epithelial-muscular cells (protection, movement)

Digestive cells (phagocytosis and digestion of food)

Glandular cells (secretion of digestive juice)

Digestion is abdominal and intracellular.

reproduction asexual - budding in the warm season, sexual- with the onset of autumn cold. Hermaphrodites. Spermatozoa enter the water and merge with a mature egg located 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 parts of the body, thanks to the reproduction and differentiation of non-specific (intermediate) cells.

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

hydroid polyps Hydroidea

The development cycle of hydroid polyps consists of an alternation of two generations, different in structure and method of reproduction. (Fig. 2.4).

Polyps - The first generation leads a sedentary lifestyle and breeds only asexually, producing through the budding of polyps and jellyfish.

Jellyfish(second generation) break away from polyp colonies and move to a free mobile lifestyle (hydromedusas). They reproduce sexually and again give rise to a generation of polyps.

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

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

Being at first a free-moving independent generation, jellyfish, thus, gradually become, as it were, the genital organs of a colony of polyps: interesting example reduction of the individual to the level 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. The burns of many jellyfish are sensitive to large animals and humans.

Between the ectoderm and endoderm there is a gelatinous mesoglea. Along the edges of the umbrella are tentacles surrounding the mouth, located on the underside. The mouth leads to the gastric cavity, from which radial canals depart, which form the gastric system. The stomach cavity is divided into chambers.

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

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

Along the edge of the umbrella are complex sense organs - ropalia, each of which contains an "olfactory fossa", an organ of balance and stimulation of the movement of the umbrella - statocyst, photosensitive eye.

Rice. 2.5. The structure of a jellyfish and a polyp

Jellyfish have separate sexes. Sex cells are formed in the sex glands - gonads located in the endoderm.

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

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

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

Rice. 2.6. Development cycle of scyphomedusa

Class 3. Coral polyps

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

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

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

Gonozoid It is a rod-shaped formation elongated in length with an expansion 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 a blastostyle. The skeletal membrane gives an extension around the blastostyle - the gonotheca. All this formation as a whole is called gonangia. Jellyfish are formed in them by budding. They bud from the blastostyle, emerge from the gonangium, and begin to lead a free lifestyle. As the jellyfish grows, germ cells form in its gonads, which are released into the external environment, where fertilization occurs.

A blastula is formed from a fertilized egg (zygote), with the further development of which a two-layered larva, planula, freely floating in water, covered with cilia, is formed. Planula settles to the bottom, attaches 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. The structure of the coral polyp

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

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

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

Rice. 2.8. reef coral polyps

Eight-ray 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-ray corals there are non-skeletal forms - sea ​​anemones (Fig. 2.9). These are large single coral polyps, devoid of a skeleton. They are capable of slow movement with the help of 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, mollusks, etc.

Anemones, settling on empty mollusk shells inhabited by hermit crabs, form an interesting symbiosis with them: the cancer moves the anemone along the bottom to better places for hunting and delivers food to it - the remnants of its meal, while the anemone protects the cancer from enemies with the help of stinging cells (Fig. 2.10).

Rice. 2.9. sea ​​anemone

Rice. 2.10. Symbiosis of sea anemone with hermit crab

Another species of sea anemone has a symbiotic relationship with clown fish. The brightly colored fish, immune to the poison of the tentacles, lure enemies, and the sea anemone grabs them and eats them. (Fig. 2.11).

Rice. 2.11. Clownfish among the tentacles of anemones

Ecology

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

Type flatworms

General characteristics of the type

The external structure of the body

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

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

dimensions from 2 cm to several meters

symmetry bilateral (aromorphosis type)

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

The 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 intestinal animals, but three germ layers. Between the ectoderm, which forms the integument, and the endoderm, from which the intestines are built, they have an intermediate germ layer - mesoderm. In the process of evolution, tissues appeared: muscular, connective, epithelial and nervous (type aromorphoses).

body cavity

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

Digestive system

Mouth on the ventral side of the body. Digestion is intracellular, partly in the intestinal cavity. There is no anal opening, so undigested food remains are removed through the mouth.

Rice. 2.19. The internal structure of flatworms

Circulatory system

Absent

Respiratory system

excretory system

protonephridial tubular system. These tubules from the parenchyma of the body end in a terminal cell with a bundle of cilia, which is called fiery 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 system of tubules (type aromorphosis).

Nervous system

The head ganglion, from which two main nerve trunks with numerous nerves depart (type aromorphosis).

sense organs

reproduction

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

Development

Regeneration

In some species

Value in nature

positive

They are an important link in the food chain of 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 they lift is deposited on the corals, which also leads to their death.

Systematics

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

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

View Planaria dairy

Ecology Habitat: fresh water. Predatory animals

Free-swimming aquatic animals with the help of cilia. There is anterior and posterior ends of the body, dorsal and ventral sides (Fig. 2.20). Covers are formed by ciliary epithelium. Sizes up to 2 cm.

The development is direct. Fertilized eggs are laid in a cocoon where small planarians develop. Regeneration well developed.

Rice. 2.20. Dairy planaria

Rice. 2.21. Features of the structure of dairy planaria

Species Liver fluke

The 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)

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

body cavity absent. filled with parenchyma.

Digestive system

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

excretory system

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

Nervous system

Periopharyngeal nerve ring, from which 2-3 pairs of nerve trunks extend, interconnected by nerves.

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

reproduction

The reproductive system is very complex, as the flukes hermaphrodites. Fertilization is internal. Possible self-fertilization.

Rice. 2.22. The internal structure of the liver 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 definitive host. For further development, eggs must necessarily fall into a fresh pond.

2) In the water, a larva with cilia comes out of the egg miracidium. Which swims for a while, then finds an intermediate host of a freshwater gastropod Prudovik small and is introduced into his liver.

3) In the liver of a mollusk miracidium develops into the next larval stage - sporocyst.

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

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

7) Cattle eat grass with adolescarii.

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

Thus, the only way to infect the definitive host is ingestion adolescarians (Fig. 2.23).

Rice. 2.23. Life cycle of the liver fluke

View Bull tapeworm

The external structure of the body

Development cycle

The eggs develop into larvae oncosphere with three pairs of hooks on the posterior pole, which turns into finnu (cysticercus). This is the blistering stage of development tapeworm, is always located in the body of the intermediate host. Finna - a vial filled with liquid, with a head screwed inside, the size of a grain of rice.

Infection of a person occurs when using 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. Development cycle bull tapeworm

General characteristics of the type

body cavity

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

Features of the internal structure (Fig. 2.30)

Three layer animals. The body is not divided into segments. Skin-muscular sac with 4 strands of longitudinal muscles. Roundworms do not have transverse muscles, so they cannot shorten and lengthen the body.

Respiratory system

Digestive system

The presence of a mouth, pharynx, stomach, the appearance of the hindgut and anus, which made it possible to make the process of digestion phased

circulatory system

Absent, the cavity fluid performs the transport function.

excretory system

« skin gland”, represented by secretory cells located in the front of the body. One or two channels 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 pore. 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 in the cytoplasm the residual metabolic products.

Nervous system

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

sense organs

reproduction

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

The emergence of dioecy and internal fertilization ensured combinative variability and genetic diversity of offspring.

Development cycle

Value in nature

Significance in human life

Rice. 2.30. The internal structure of roundworms

Class 2. Cynorrhinhas

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

Rice. 2.32. Cynorrhinha

Class 3. Hairy

Class 4. Nematodes

Species Human roundworm

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, highlands, deserts and semi-deserts. Most often it occurs in residents of the tropical and subtropical zones, where the prevalence of ascaris in the population reaches 60 - 80%. Roundworms have been known to people for a long time, as evidenced by the mention of this helminth in the writings of Hippocrates.

According to World Organization Health (WHO), in the world about 1 billion people are affected by ascarism. The structure of the body is spindle-shaped, non-segmented, round in cross section. At the front end of the body is a mouth with three lips. Length 20-25 cm males, 40 cm females. Color yellowish yellow (fig.2.33).

Rice . 2.33. Human roundworm

The skin-muscle sac consists of skin covered with a cuticle, under which there is a non-cellular hypodermis. Longitudinal muscles are attached to the skin. Absent. Respiration is anaerobic (glycolysis). The sense organs are tactile tubercles and pits. Reproduction is sexual. Separate animals. Sexual dimorphism (i.e. females and males look 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 over 200,000 eggs per day.

Development cycle

Development occurs when a person swallows roundworm eggs while eating unwashed vegetables or fruits, or when personal hygiene is not followed. Development goes without change of owners.

Eggs are covered with several protective shells and are able to remain viable for up to 10 years. Further development must pass through the soil optimum temperature 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, larvae capable of developing in the human body are formed in the egg within 15–20 days. In the human gastrointestinal tract, they come out of the eggs.

With the blood stream, the larvae enter the liver, and then to the lungs. Here they develop. The larvae are then coughed up into the throat and 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 effect of adults: mechanical, toxic-allergic effect, mutagenic, metabolic disorders.

Symptoms of ascariasis:With pastic cough, disorders of the gastrointestinal tract, weakness

Rice. 2.34. Roundworm development cycle

human pinworm

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

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

The Animal Kingdom has a number of distinctive features:

1. Heterotrophic type of nutrition. Most have holozoic, some have osmotrophic, phago- and pinocytosis. Some mixotrophs (euglena green).
2. Specific features in the organization animal cell: does not have a cell wall (therefore, it can take different shape), the vacuole system is not developed, there are centrioles, many cells are equipped with cilia or flagella, the main reserve substance- glycogen.
3. Four types of fabrics: epithelial, connective, muscular and nervous.
4. Basically mobile lifestyle related to the development musculoskeletal And nervous systems.
5. Available excretory organs and excrete nitrogen-containing waste products (ammonia, urea, uric acid, etc.).
6. For the higher are characteristic complex behavioral responses. Highly organized forms are able to carry out processes of higher nervous activity.
7. Most have nervous And humoral regulation system(in plants only humoral).
8. Available defense (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 simpler than 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 Animal Kingdom is divided into two sub-kingdoms: Unicellular and Multicellular.
Subkingdom Unicellular includes types: Sarcomastigophora (classes Sarkodovye and Flagellates), Ciliates (class Ciliary ciliates), Apicomplex (class Sporoviki).
Subkingdom Multicellular includes types: Coelenterates (classes Hydroid, Scyphoid and Coral polyps), Flatworms (classes Flukes, Tapeworms, Ciliary worms), Roundworms (class Proper roundworms, or Nematodes), Annelids (classes Small-bristle, Polychaete and Leeches), Mollusks (classes Gastropods, Bivalves, Cephalopods), Arthropods (classes Crustaceans, Arachnids and Insects), Chordates. Type Chordates divided into three subtypes: Tunicates (class Ascidians), Cranial (class Lancelets), Vertebrates (classes cartilaginous fish, Bony fish, Amphibians (Amphibians), Reptiles (Reptiles), Birds, Mammals).

Subkingdom Protozoa (single-celled)

general characteristics

Type Sarcomastigophora

Rhizome class (Sarcode)

Type of Infusoria

Class Ciliary ciliates

Type coelenterates

general characteristics

There are about 9 thousand known species of coelenterates. Habitat - aquatic (marine reservoirs with the exception of a few freshwater species). Lifestyle - free-living: free-floating or attached forms.
Systematics. Type Intestinal includes classes: Hydroid, Scyphoid and Coral polyps.
Structure. Most coelenterates are characterized by two life forms: an attached polyp and a free-swimming jellyfish. In many, both forms alternate during the life cycle (polyps - asexual generation, jellyfish - sexual).
Polyp(attached form) has the form of an elongated bag with a hole - the 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 is a mouth surrounded by oral lobes. Tentacles are located along the edge of the dome. Floating forms are always single.
Body sizes from 1 mm to 2 m. Coelenterates have radiant (radial) type of symmetry, 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 support plate that acts as an internal skeleton (in coral polyps). Coral polyps and colonial hydroids, in addition, have an external calcareous or horny skeleton.
The cells of the ectoderm and endoderm are differentiated according to their functions.
Ectoderm cells. The ectoderm includes epithelial-muscular, stinging, nerve, intermediate and germ cells.
Endoderm cells. The endoderm includes epithelial-muscular, glandular, nerve and sex cells.
Epithelial-muscular cells line the gastric cavity, have 2–5 flagella, muscle fibers (located perpendicular to the longitudinal axis of the body) are able to form pseudopods. They provide the movement of water in the gastric cavity and intracellular digestion.
glandular cells produce and secrete digestive enzymes into the intestinal cavity, providing abdominal digestion.
nervous the cells are analogous to the nerve cells of the ectoderm.
Sexual(in scyphomedusa) are similar to the germ cells of the ectoderm.
Movement It is carried out due to the contraction of the muscle fibers of the epithelial-muscular 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. Single polyps (hydra) move by "tumbling", jellyfish - in a jet 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. Mobile forms have organs of vision (eyes) and balance ( statocysts- bags with pebbles made of carbonic lime inside).
Digestion. Most coelenterates actively capture food with tentacles. For the attack, stinging cells are used, which paralyze the victim. Food enters the digestive (gastric) cavity through the mouth, where it is digested. There are two types of digestion: intracellular and abdominal. Intracellular Digestion is carried out by epithelial-muscular cells of the endoderm, which capture food particles by endocytosis. Cavitary Digestion is possible due 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 by a stream of water.
Respiration and excretion products of metabolism is carried out by the entire surface of the body.
Regeneration- restoration of lost or damaged body parts. It is possible due to the reproduction and differentiation of intermediate cells.
Reproduction. Most are dioecious. Some hydroids - hermaphrodites They have both ovaries and testes. There is an alternation of asexual and sexual reproduction. Asexual reproduction occurs by budding or strobilation. budding- reproduction by the formation of a kidney on the mother's body - 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 organism. The development is direct. In jellyfish and marine hydroids, germ cells are released into the water, where fertilization occurs. Development with metamorphosis, larva - planula.
Origin and aromorphoses. The following aromorphoses led to the emergence of the type: cell differentiation and tissue formation, diffuse-type nervous system, abdominal digestion.
Meaning. Coelenterates are an important link in the food chains of marine animals, they contribute to water purification (biological filter feeders). Some types of jellyfish are poisonous (cyanoea, cross), some are used for food. Coral polyps form the unique ecological systems of coral reefs. In the same time Coral reefs and islands (atolls) make navigation difficult. Limestone deposits used in construction are formed from the skeletons of coral polyps.

Class Hydroids

Life forms - polyp (freshwater hydra) or polyp and short-lived jellyfish (obelia).
Freshwater hydra. Habitat - fresh water. Free-living, attached. The body length is about 1 cm. The body consists of a bag-shaped body, soles and tentacles. Attaches to the substrate with the sole. The body is double layered. The mouth is surrounded by tentacles (5–12) that serve to capture food. Ectoderm cells: epithelial-muscular, nervous, stinging, intermediate, genital. Endoderm cells: epithelial-muscular, glandular and nervous. With the help of epithelial-muscular cells, the body is able to move. Stinging cells serve for defense and attack. Breathing is carried out by the entire surface of the body. The nervous system is of a diffuse type, consisting of nerve cells scattered throughout the body. Developed sense of touch. The gastric cavity has no partitions and channels. Asexual reproduction (budding) occurs in summer. Sexual reproduction occurs in autumn. In the ectoderm, the sex glands are formed, where gametes are formed (spermatozoa with flagella and an amoeboid egg), fertilization occurs on the body of the mother hydra. The medusoid form is absent. The development is direct.
hydroid polyps(obelia). The alternation of asexual and sexual generations (metagenesis) is characteristic. The 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 the colony, later they separate from it and lead a free lifestyle. Reproduction in hydroid jellyfish is sexual. Insemination is external (sex cells are released into the water). Development with metamorphosis (larva - planula).

Class Scyphoid

Scyphoid jellyfish s (cornerot, cyanide, gonionema). They live only in the seas. The medusa stage predominates over the polyp stage. The jellyfish resembles an inverted and strongly flattened polyp. The content of the jellyfish is represented by a 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 due to the expulsion of water from under the dome while reducing the walls of the umbrella. Separate sexes. Characteristic alternation of generations. Polyp reproduction occurs strobilation- ordered transverse division of the polyp into several parts. Sex cells are formed in the endoderm. A larva develops from a fertilized egg. After attaching to the substrate, a polyp develops from it. Growing up, the polyp begins to bud young jellyfish.

Class Coral polyps

coral polyps(anemone, horn coral, red coral). They exist only in the form of a polyp. They live in the shallow waters of 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 tentacles (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. Reproduction is asexual (budding) and sexual. Dioecious, sex cells are formed in the endoderm. Development is direct or with metamorphosis (larva - planula). The calcareous skeletons of colonial forms form reefs and oceanic islands.

Type flatworms

general characteristics

Class Ciliary worms

White (milk) 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.

Flukes class

Class Tapeworms

Bull tapeworm. Size 4–10 m. Body shape - ribbon-like. Sections of the body - head, neck, segments(up to 1 thousand or more). The head has four suckers, the neck is undivided, the body is long, ribbon-like, dissected. The digestive system is missing. The respiratory system is missing. Anaerobe. The nervous system is poorly developed. Chains are hermaphrodites. Each segment contains one ovary and many testes. Segments containing eggs are isolated from the human intestine (the main host). Together with the grass, they enter the stomach of the cow (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 tapeworm head inside). When a person consumes uncooked Finnose meat, the head of the tapeworm attaches to the wall and begins to produce segments.
Echinococcus. The adult form is up to 6 mm long. It consists of 3-4 segments, has suckers and a proboscis with a halo of hooks on the head. Members do not separate. The main host is dogs, wolves, foxes. Their tapeworm lives in the small intestine. Intermediate - sheep, pigs, goats, cattle, deer, humans. In the intermediate host, the Finn stage develops - a bubble with many heads. Blisters develop in the lungs, liver, brain, bones and are the size of a child's head. Infection of a person occurs by swallowing tapeworm eggs that have fallen on the hands after contact with dogs and wild animals.

Type roundworms

general characteristics

Class Nematodes (Actually roundworms)

Type annelids

general characteristics

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

The segments have the same external and internal structure (metamerism) and perform similar functions. The metameric structure of the body determines the high ability to regenerate.
The body wall is formed skin-muscle 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 circular muscles, the body of the worm becomes long and thin, with the contraction of the longitudinal muscles, it shortens and thickens.
Movement organs - parapodia(available in polychaetes). These are outgrowths of the skin-muscular sac on each segment with tufts of setae. In oligochaetes, only tufts of setae are retained.
body cavity secondary - in general(has an epithelial lining covering 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 partitions, corresponding to body segments. The cavity fluid is the hydroskeleton and internal environment, it is involved in the transport of metabolic products, nutrients and reproductive products.
Digestive system consists of three sections: 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, along the abdominal vessel - from front to back. The movement of blood is carried out due to the rhythmic contractions of the walls of the spinal vessel and the annular vessels ("heart") in the pharynx, which have thick muscular walls. Many people have red blood.
Breath. Most annelids have cutaneous respiration. Polychaetes have respiratory organs - pinnate or leaf-shaped gills. These are modified dorsal antennae of parapodia or head lobe.
excretory system metanephridial type. Metanephridia have the form of tubes with funnels. Two in each segment. The funnel, surrounded by cilia, and the convoluted tubules are in one segment, and the short tubule, which opens outwards with a hole, is the excretory pore, in the adjacent segment.
Nervous system represented by supraglottic and subpharyngeal nodes ( ganglia), circumpharyngeal nerve ring (connects the supraesophageal and subpharyngeal ganglia) and abdominal nerve cord, consisting of paired nerve nodes in each segment, connected by longitudinal and transverse nerve trunks.
Sense organs. Polychaetes have organs of balance and vision (2 or 4 eyes). But most have only separate olfactory, tactile, gustatory, and light-sensitive cells.
Reproduction and development. Soil and freshwater forms are mostly hermaphrodites. Sex glands develop only in certain segments. Insemination is internal. The type of development is direct. In addition to sexual reproduction, asexual reproduction is also characteristic (budding and fragmentation). Fragmentation is carried out due to regeneration - the restoration of lost tissues and body parts. Marine representatives of the type are dioecious. The sex glands in them develop in all or in certain segments of the body. Development with metamorphosis, larva - trochophore.
Origin and aromorphoses. The following aromorphoses led to the emergence of the type: organs of movement, organs of respiration, a closed circulatory system, a secondary cavity of the body, and segmentation of the body.
Meaning. Earthworms improve soil structure and increase soil fertility. The oceanic palolo worm is eaten by humans. Medical leeches are used for bloodletting.

Class Small-bristle (Oligochetes)

Representatives: earthworms, tubules, etc. Most of the small bristles live in the soil and fresh waters. Detritivores They feed on semi-decomposed remains of plants and animals. Parapodia are absent. The setae extend directly from the body wall. The head lobe is weakly expressed. Sense organs are often absent, but there are olfactory, tactile, gustatory, photosensitive cells. Hermaphrodites. Insemination is internal, cross. Development is direct, takes place in a cocoon, which, after fertilization, is formed on the body of the worm in the form of a girdle, and then slides off it.
The role of earthworms in soil formation is enormous. They contribute to the accumulation of humus and improve soil structure, thereby increasing soil fertility.

Class Polychaetes (Polychaetes)

Leech class

type of shellfish

general characteristics

Over 130 thousand species have been described. In terms of the number of species, molluscs are second only to arthropods. Habitat: marine and fresh waters, wet land areas. Most mollusks are free-living. Protostomes. They develop from three germ layers. They lead a sedentary lifestyle.

Systematics. The phylum Mollusca includes the classes: Gastropods, Bivalves, Cephalopods.
Structure. Mollusks (soft-bodied) have a soft non-segmented body. Most are bilaterally symmetrical, while gastropods are asymmetrical. Body sizes from 2–3 mm to 18 m.
Sections of the body. The body is divided into head, leg, torso

Bivalves do not have a head. Leg is a muscular outgrowth of the abdominal wall of the body, which serves for movement. torso contains internal organs head the mouth and sense organs are located.
The body of a mollusk is usually covered 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 the 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 that completely or partially covers the body of a mollusk.
Between the mantle and the body of the mollusk is mantle cavity. It houses the organs of respiration and chemical sense and opens the digestive, excretory and reproductive system. The mantle cavity communicates with the external environment siphons(in water forms) or breathing holes(for terrestrial).
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, midgut) and posterior (hindgut, anus). There are liver, salivary glands (many). Horny jaws are located in the oral cavity. There is a tongue in the pharynx grater, or radula) covered with teeth. The hindgut opens into the mantle cavity. Mollusks eat 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 spaces between the organs - gaps. It washes the organs, then collects in vessels that go to the respiratory organs, and from there to the heart. Blood is often colorless, sometimes contains a substance similar in structure to hemoglobin.
Respiratory system. In water forms - skin gills(mantle folds), in terrestrial forms - lung(mantle pocket) with a 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 body and interconnected by nerve trunks.
sense organs represented by the organs of vision (eyes), touch, balance and chemical sense.
Reproduction and development. There are both dioecious and hermaphrodites. Reproduction is sexual. Sex glands (testes and ovaries) are paired. Insemination is 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(for bivalves).
Mollusks move with the help of their legs (wave-like muscle contractions) or reactively (pushing out water when the shell closes abruptly or through a funnel from the mantle cavity).
Origin and aromorphoses. Mollusks are descended 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 Gastropoda

Representatives: grape snails, pond snails, coils, slugs, rapans, etc. The habitat is aquatic and ground-air. They live in fresh water bodies, seas, damp places on land.
A characteristic feature is the asymmetry of the structure, due to the reduction of the organs of the right side and the predominant development of the organs of the left side. The shell is entire, 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. On the head are one or two pairs of tentacles. At their base or at the ends of the first pair are the eyes. There are both herbivorous snails (they feed by scraping algae or tissues of higher plants - a pond snail, a coil, a grape snail), and predatory forms (rapana eat mussels, oysters).
Meaning. Man uses 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 (rapana) harm oyster and mussel settlements.

Class Bivalve

Class Cephalopoda

Representatives: octopuses, squids, cuttlefish, etc. Highly organized molluscs. They live mainly in warm seas and oceans. All predators. The jet mode of movement is characteristic.
The body consists of a head and a body. Leg converted to tentacles (arms) surrounding the mouth opening. The shell is internal, often reduced or absent. Available cartilaginous "skull" and two thick horny jaws (beak), which captures and crushes food. Cephalopods have two pairs of salivary glands, the secretions 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 hindgut 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 and usually reproduce once in a lifetime. The development is direct.
Meaning. Fishing object (cuttlefish, squid, octopus). Source of pharmaceutical raw materials. From the secret of the ink sac of cuttlefish and squid, Chinese ink and sepia watercolor are obtained.

Type arthropod

general characteristics

body integuments presented cuticle And hypodermis. The skin-muscular sac, characteristic of the previous groups, is reduced, which is associated with the presence of a dense outer cover. The cuticle is made up of chitin. Chitin can be impregnated with lime salts (shell of higher crustaceans) or proteins (insects). The chitinous cover performs a protective function - it protects against drying and mechanical influences. Thanks to him, arthropods were the first of the animals to inhabit the land. In addition, the chitinous cover is the outer skeleton - to its inner surface bundles of striated muscles are attached. The appearance of this type of musculature provided an increase in mobility. The chitin cover is inextensible, so the growth of arthropods is accompanied molting.
organs of movement. In primitive arthropods, each body segment has a pair jointed 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 specific function and were transformed into sensory organs, mouth organs, walking and swimming limbs, gills, spider warts, etc.
body cavity mixed - mixocell. It is formed by the confluence of sections of the primary and secondary cavity.
Digestive system has three sections - anterior (mouth, pharynx, esophagus, sometimes goiter), middle (stomach, midgut) and posterior (hindgut and anus). The anterior and posterior sections have a cuticular lining. There are liver and salivary glands. A complex structure appears oral apparatus from modified forelimbs. It is specialized for a certain type of food (gnawing, licking, sucking, piercing-sucking, etc.).
Circulatory system open. Available heart located on the dorsal side of the body. Hemolymph circulates through the vessels. It is a colorless liquid that has a dual nature: partly corresponding to the blood, partly to the cavity fluid. From vessels hemolymph pours into the body cavity and washes the internal organs. Then it again enters the vessels and the heart.
Respiratory system. Primary aquatic arthropods have gills, for terrestrial lung sacs And trachea(chitinous tubes penetrating the entire body).
excretory system represented by modified metanephridia ( green and coxal glands), fat 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 fat body.
Nervous system consists of the supraesophageal and subpharyngeal nerve nodes (ganglia), connected by nerve cords into the peripharyngeal ring, and the abdominal nerve chain.
Sense organs: vision, taste, touch, smell, hearing and balance.
Reproduction and development. Usually segregated. Sexual dimorphism is well expressed. The female has ovaries and oviducts, the male has testis, vas deferens and ejaculatory canal. Reproduction is only sexual, parthenogenesis and live birth occur. Development can be direct, with complete or incomplete metamorphosis. Growth is possible only with periodic molting - dropping the old cuticle and the formation of a new one.
Origin and aromorphoses. Arthropods evolved from ancient marine annelids. The following aromorphoses led to the emergence of the type: the appearance of the external skeleton, jointed limbs, and striated muscles.

Class Crustaceans

Limbs. The cephalothorax and abdomen consist of unequal segments, each of which corresponds to a pair of jointed limbs specialized to perform a specific function. Crayfish has the following limbs: segments cephalothorax carry 13 pairs of limbs: antennules(olfactory organs) antennas(organs of touch), upper jaws and 2 pairs of lower jaws (grinding food); 3 pairs of mandibles (supplying food to the mouth) and 5 pairs of walking legs (movement), the 1st pair of walking legs is transformed into claws (defense and attack); on 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 segment of the abdomen, form the caudal fin.

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

The oldest and very common hypothesis among zoologists is that colonial organisms, similar to colonial flagella, were transitional to multicellular forms. Among these organisms there are also 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 divided but not dispersed cells. In this case, it is assumed that at first the colonies consisted of identical cells, and then 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 crushing 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 holozoic 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 - the blastea - arose as a result of the accumulation of gelatinous matter 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, gastrea developed by invagination of the anterior wall of the blastea. Outside, the cells of the gastrea continued to carry flagella, which ensured its movement. The inner layer of cells lost flagella and turned into the primary intestine. The place of invagination gave the primary mouth, with the help of which the intestinal - gastric - cavity communicated with the external environment. Digestion of food took place in the gastric cavity.

The outer layer of the gastrea gave its descendants the ectoderm, the inner layer gave the endoderm. Thus, according to Haeckel's theory, all multicellular animals, including sponges, descended from one ancestral 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 supporters and for a long time dominated science, but at the same time caused fair criticism.

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

One of the serious opponents of this theory was II Mechnikov. His most significant objections to Haeckel were the following: 1. The formation of gastrula by invagination cannot be considered primary, since in the most primitive multicellular organisms (intestinal, intestinal turbellarians) 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 the lower multicellular intestines are characterized by intracellular digestion to a large extent. 3. The process of intussusception in phylogenesis could not be due to either physiological or ecological reasons. II Mechnikov suggested 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 capturing food particles, the cells lost their flagella and left the surface for the interior of the body. Digestion of food took place there, after which the cells could again return to the surface and form a new flagellum. Thus, there was a primary, optional (temporary) isolation of the outer layer of cells - the kinoblast, which have the function of movement, and the inner mass of cells - the phagocytoblast, involved 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 I. I. Mechnikov later).

The hypothesis of I. I. Mechnikov was based on a large amount of material from his own research on the embryology of lower multicellular organisms (sponges and coelenterates). He first set important issue the evolution of ontogenesis itself, changes in the methods of gastrulation and cell differentiation in different groups of lower coelenterates. He made many new contributions to the theory of primary germ layers and their evolution.

Relatively recently, another hypothesis for the origin of multicellularity, called the polyenergy or cellularization hypothesis, has been put forward. Its author is the scientist I. Hadji. At first, he considered the ancestors of multicellular animals to be multinuclear flagellates, with a large number of flagella, and later ciliate forms (ciliates, before their nuclear dualism). From them, according to Hadji, two branches of the animal world went - one to modern ciliates, the other to the most primitive (in his opinion) multicellular, intestinal-less ciliary worms (Acoela). I. Hadji compared the structure of ciliates and intestinalless turbellarians and found many external similarities between them. From this, he concludes that the organelles of protozoa turned into multicellular organs, while an increase (multiplication) in the number of nuclei and the subsequent separation of plasma around them (cellularization) 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 the 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 by no means in all species. Recently managed to confirm true cellular structure Acoela; using an electron microscope, cell membranes were found in their outer epithelium.

Comparative analysis body structure of ciliates and non-intestinal turbellarians showed that it is impossible to draw true homologies between these groups of organisms. In addition, all embryological material is in conflict 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 literary data. He comes to the conclusion that the most convincing hypothesis of the origin of multicellular organisms is the hypothesis of phagocytella by I. I. Mechnikov.

The ancestors of multicellular (Metazoa), apparently, were heterotrophic collared flagellates (Craspedomonadina) from the order of protomonads (Protomonadida). From a spherical free-floating colony consisting of identical flagellates, more complex colonies arose with greater integration. Initially, reproduction was asexual, the colony broke up 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 sexual. Simultaneously, the differentiation of the anteroposterior axis of the colony and the definition of its anterior and posterior ends (poles) took place. The radial symmetry of the colony acquired a multipath character.

Initially, the germ cells - gametes - were the same and isogamous copulation was observed, and later the differentiation of male and female gametes occurred and anisogamy arose. A fertilized egg - a zygote - began to intensively divide up to since then until a new blastula-like colony emerges.

Further differentiation of the colony led to its transformation into an independent organism, similar to phagocytella. At the same time, at first a temporary, or optional, and then a permanent isolation of the outer layer, or kinoblast, and the inner, or phagocytoblast, occurred (according to I. I. Mechnikov). The resulting organism, 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 the further differentiation of its cells. At the same time, part of the cells left the surface of the larva inward, forming the inner layer. Thus, a two-layer structure of phagocytella arose. Then germ cells emerged from the somatic cells and the puberty of the organism arose.

A. V. Ivanov suggests the formation of a mouth opening at the posterior pole of the phagocytella as a further stage of development. Initially, its amoeboid phagocytes approached the surface anywhere and captured food particles. However, with the emergence 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. This is where the mouth opening appears, through which it is easier for phagocytes to capture food. This circumstance is consistent with the actual material and explains the formation of the primary mouth in all multicellular organisms at the posterior, vegetative pole of the embryo.

AV Ivanov considers sponges and intestinalless turbellarians to be the closest forms to the original common ancestor of all Metazoa - phagocytella.

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

The second branch of development goes to the common ancestor of two-layered animals, from which two types then descended - intestinal (Coelenterata) and ctenophores (Ctenophora). In the beginning, these forms were floating. An attached lifestyle 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 that retained the primary mode of locomotion due to the cilia of the rowing plates, homologous to the phagocytella kinoblast.

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

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

V. N. Beklemishev, who also shares the phagocytella hypothesis of I. I. Mechnikov, draws attention to the great similarity (in the main features of organization) of adult ctenophores and turbellarians with intestinal larvae. He explains this similarity by the common origin of both groups (ctenophores and turbellarians) from more or less close ancestors. According to V.N. Beklemishev, ctenophores and turbellarians have a shortened life cycle compared to intestinal. He suggests that ctenophores and turbellarians developed from the ancestors of the coelenterates by neoteny, that is, from their larval forms that passed to progressive evolution. The further development of these groups went to some extent in parallel, or convergent, which is manifested in the similarity of the structure (symmetry) nervous system and in the formation of an aboral statocyst. However, ctenophores formed as planktonic (with rare exceptions) forms, while turbellarians formed as benthic ones. As a result of crawling along the bottom, they developed bilateral symmetry.

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