Flatworms belong to the group of three-layered animals. In addition to the ecto- and endoderm, the embryos of flatworms develop a third germ layer - mesoderm. During development, these three leaves form the tissues and organs of the worms' body.

Flatworms have bilateral (two-sided) symmetry; only one plane can be drawn through their body, dividing the body into symmetrical halves. With bilateral symmetry, the body is distinguished between right and left halves: ventral and dorsal sides, anterior (head) and posterior (tail) ends. These signs are a consequence of aromorphoses that occurred in the ancestors of flatworms. Flatworms are protostomes.

The body of flatworms has a leaf- or ribbon-like shape and is always flattened in the dorsoventral direction, which gave rise to the name of the type. The body wall is formed by a skin-muscle sac. It consists of a layer of epithelium that covers the outside of the body, and continuous layers of muscle underlying it. The outer layer is represented by circular muscles, the inner layer is longitudinal. Between them there are usually diagonal muscles. Contraction of the muscular elements of the skin-muscular sac provides the characteristic “worm-like” movements of flatworms.

The internal organs are immersed in loose connective tissue of mesodermal origin - parenchyma containing numerous cells. The functions of the parenchyma are diverse: it has a supporting role, serves to accumulate reserve nutrients, and plays a role in metabolic processes. Since parenchyma fills the space between organs, flatworms are called cavityless, parenchymatous animals. They do not have a body cavity.

Excretory system in flatworms it is represented by excretory organs - protonephridia. Their function is to remove intracellular breakdown products (dissimilation products) from the body. The latter are excreted from all cells of the body and enter the intercellular spaces of the parenchyma. From here they are extracted by special cells with a “flickering flame”, i.e. with a bunch of eyelashes. Inside these cells, the tubules of the excretory (excretory) system begin. The beating of the cilia drives waste products through the tubules. Coming together, these tubules form increasingly larger tubes that flow into the paired (right and left) canals of the excretory system, which merge together and open outward into the excretory pore.

Flatworms are hermaphrodites. The reproductive system consists of the gonads (testes and ovaries) and complex system channels that remove reproductive products.

Animals belonging to the type of flatworms are characterized by:

1. three-layer, i.e. development of ecto-, ento- and mesoderm in embryos;
2. the presence of a skin-muscle sac;
3. absence of a body cavity (the space between organs is filled with parenchyma);
4. two-sided symmetry;
5. body shape, flattened in the dorsoventral (dorsoventral) direction;
6. the presence of developed organ systems: muscular, digestive, excretory, nervous and reproductive.

The phylum flatworms (Plathelminthes) includes 6 classes. Will be discussed here

• Class ciliated (Turbellaria)
• Class flukes (Trematodes)
• Class tape (Cestoidea)

Class ciliated (Turbellaria)

About 1,500 species of ciliated worms or turbellarians are known. Turbellaria are common in all parts globe. Most species live in the seas, where flatworms apparently first arose. Freshwater and soil species are known. Almost all turbellarians are predators. They eat protozoa, worms, small crustaceans and insects. There are intestinal forms, as well as species with straight and branched intestines. Typical representatives The ciliated worms are planarians.

A small (10-15 mm long) leaf-shaped worm that lives in ponds and low-flowing reservoirs. Planaria can be found on pieces of wood rotting underwater, fallen tree leaves and plant stems.

Body coverings and movement apparatus. The body is covered with cilia. The body wall of planaria, like all flatworms, is formed by skin and muscles, which, tightly fused, make up a skin-muscular sac. Single-celled mucous glands are developed in the skin. Muscles are represented by fibers arranged in three layers (circular, oblique and longitudinal). This allows planarians to move and change their body shape somewhat.

No body cavity. Inside the skin-muscle sac between the organs there is spongy parenchyma tissue, consisting of a mass of cells, the small spaces between which contain tissue fluid. It is associated with the movement of nutrients from the intestines to all parts of the body and the final waste products to the excretory organs.

Digestive system. The mouth is located on the ventral side, in the middle or in the posterior third of the body. The digestive system consists of an anterior section - the ectodermal pharynx, and a middle section, which has the appearance of highly branched trunks ending blindly. Undigested food remains are expelled through the mouth. In ciliated worms, along with extracellular digestion, intracellular digestion plays an even greater role. Some planarians have no intestine and digestion is carried out only by phagocytic cells. Aintestinal tubellaria are of considerable phylogenetic interest (see below).

Excretory system. Protonephridia begin deep in the parenchyma as stellate-shaped terminal or terminal cells. The terminal cells contain tubules with a bunch of cilia that oscillate like a candle flame. Hence their name - flickering, or ciliary, flame. Terminal cells flow into tubules, the walls of which already consist of many cells. These tubules are numerous and permeate the entire body. They open into lateral canals with a large lumen, and finally communicate with the external environment through excretory pores. Protonephridia perform the functions of osmoregulation and removal of dissimilation products from the body. Terminal cells absorb tissue fluid from the parenchyma. The flickering flame promotes its movement through the channels to the excretory pore.

Nervous system. In ciliated worms, at the head end there is a paired cerebral ganglion and nerve trunks coming from it, of which two lateral trunks, consisting of nerve cells and their processes, reach the greatest development. The transverse trunks are connected by ring bridges, due to which the nervous system takes on the appearance of a lattice.

Sense organs still primitive. They are represented by tactile cells, which the skin is rich in, one or more pairs of pigmented ocelli, and in some, balance organs - statocysts.

Reproduction. Planarians are hermaphrodites with a complex reproductive system. They have widely developed asexual reproduction and well-defined somatic embryogenesis. Thanks to this, they serve as classical objects for studying regeneration processes.

Origin. The question of the origin of eyelash worms has not been completely resolved. The most widely accepted hypothesis is V.N. Beklemisheva (1937). He believes that the oldest turbellarians are the intestinal ones. According to his hypothesis, they originated from the planula-shaped (i.e., similar to the planula - the larva of coelenterates) ancestor of flatworms, which switched to crawling. This way of life contributed to the separation of the dorsal and ventral sides of the body, i.e., the formation of bilateral symmetry.

According to the hypothesis of A.V. Ivanov (1973), the lower intestinal turbellariae evolved directly from the phagocytella, bypassing the coelenterates. According to his concept, coelenterates are a side branch of the animal world.

Initial level of knowledge:

Response plan:

• General characteristics of flatworms
• External and internal structure Flatworms
• Reproduction of Flatworms
• Classification of Flatworms, variety of species
• Peculiarities of the structure and development of worms of the Ciliate class using the example of the Milk planaria
• Features of the structure and development of worms of the class Flukes using the example of the Liver fluke
• Features of the structure and development of worms of the Tapeworm class using the example of the Bull tapeworm and others.

General characteristics of flatworms

Number of types: about 25 thousand.

Habitat: They live everywhere in humid environments, including the tissues and organs of other animals.

Structure: Flatworms are the first multicellular animals in which, during the course of evolution, bilateral symmetry, three-layer structure, and real organs and tissues appeared.

Bilateral(bilateral) symmetry - this means that an imaginary axis of symmetry can be drawn through the animal’s body, while right side the body will be mirror-like to the left.

During embryonic development in three-layer animals have three layers of cells: outer - ectoderm, average - mesoderm, internal – endoderm. From each layer certain organs and tissues develop:

the skin (epithelium) and the nervous system are formed from the ectoderm;

from the mesoderm - muscle and connective tissue, reproductive and excretory systems;

from the endoderm - the digestive system.

In flatworms, the body is flattened in the dorso-abdominal direction, there is no body cavity, the space between internal organs filled with mesoderm cells (parenchyma).

Digestive system includes the mouth, pharynx and blind intestine. Absorption of food and excretion of undigested residues occurs through the mouth. Tapeworms have a completely absent digestive system; they absorb nutrients over the entire surface of the body, being in the intestines of the host.

excretory organs – protonephridia. They consist of thin branching tubules, at one end of which there are flame (flickering) cells star-shaped, immersed in the parenchyma. A bunch of cilia (flickering flame) extends inside these cells, the movement of which resembles the flickering of a flame (hence the name of the cells). Flame cells capture liquid decay products from the parenchyma, and cilia drive them into the tubule. The tubules open on the surface of the body as an excretory pore, through which waste products are removed from the body.

Nervous system ladder type ( orthogon). It is formed by a large head paired nerve ganglion (ganglion) and six nerve trunks extending from it: two on the ventral side, two on the dorsal and two on the sides. The nerve trunks are connected to each other by jumpers. Nerves extend from the ganglion and trunks to organs and skin.

Reproduction and development:

Flatworms are hermaphrodites. Sex cells mature in the sex glands (gonads). A hermaphrodite has both male glands - testes, and female glands - ovaries. Fertilization is internal, usually cross-fertilization, i.e. worms exchange seminal fluid.

CLASS cilia worms

Milk planaria, a small aquatic animal, the adult is ~25 mm long and ~6 mm wide, with a flat, milky white body. At the front end of the body there are two eyes that distinguish light from darkness, as well as a pair of tentacles (chemical sense organs) necessary for searching for food. Planarians move, on the one hand, thanks to the work of the cilia covering their skin, and on the other hand, thanks to the contraction of the muscles of the skin-muscular sac. The space between the muscles and internal organs is filled with parenchyma, in which they meet intermediate cells, responsible for regeneration and asexual reproduction.

Planarians are predators that feed on small animals. The mouth is located on the ventral side, closer to the middle of the body, from it comes a muscular pharynx, from which three branches of a closed intestine extend. Having captured the victim, the planaria sucks out its contents with its throat. Digestion occurs in the intestines under the action of enzymes (intestinal), and intestinal cells are able to capture and digest pieces of food (intracellular digestion). Undigested food remains are removed through the mouth.

Reproduction and development. Ciliated animals are hermaphrodites. Cross fertilization. Fertilized eggs fall into a cocoon, which the worm lays on underwater objects. Development is direct.

CLASS FLUKES

CLASS TAPPEWORMS

Bull tapeworm– a tapeworm, reaches a length of 4 to 12 meters. The body includes a head with suckers, a neck and a strobila - a band of segments. The youngest segments are located at the neck, the oldest are sacs filled with eggs, located at the posterior end, where they come off one by one.

Reproduction and development. The bovine tapeworm is a hermaphrodite: each of its segments has one ovary and many testes. Both cross-fertilization and self-fertilization are observed. The posterior segments, filled with mature eggs, open and are excreted with feces. Cattle (intermediate host) can swallow eggs along with grass; in the stomach, microscopic larvae with six hooks emerge from the eggs, which enter the blood through the intestinal wall and are carried throughout the animal’s body and carried into the muscles. Here the six-hooked larva grows and turns into Finn- a bubble containing the head of the tapeworm with its neck. A person can become infected with finches by eating undercooked or undercooked meat from an infected animal. In the human stomach, a head emerges from the finca and attaches to the intestinal wall. New segments bud from the neck - the worm grows. Bovine tapeworm secretes toxic substances that cause intestinal disorders and anemia in humans.

Development pork tapeworm has a similar character, its intermediate host, in addition to pigs and wild boars, can also be humans, then finches develop in its muscles.

Development broad tapeworm is accompanied by a change of two intermediate hosts: the first is a crustacean (Cyclops), the second is a fish that has eaten the crustacean. The definitive host may be a person or a predator that eats the infected fish.

New concepts and terms: mesoderm, skin-muscle sac, tegument, hypodermis, reduction, protonephridia (flame cells), orthogon, strobila, ganglion, gonads, hermaphrodite, direct and indirect development, final and intermediate host, miracidium, cercaria, finna, segment, armed and unarmed tapeworm.

Literature:

1. Bilich G.L., Kryzhanovsky V.A. Biology. Full course. In 3 volumes - M.: LLC Publishing house"Onyx 21st century", 2002
2. Pimenov A.V., Pimenova I.N. Zoology of invertebrates. Theory. Assignments. Answers: Saratov, OJSC publishing house "Lyceum", 2005.
3. Chebyshev N.V., Kuznetsov S.V., Zaichikova S.G. Biology: a guide for applicants to universities. T.2. – M.: Publishing House LLC New Wave", 1998.

Flatworms are simple, segmented, soft-bodied, invertebrate, bilaterally symmetrical, which do not have space between their organs (body cavities). There are up to 25,000 thousand species in this group of worms. More than 3,000 flatworms are found in Russia. They mainly infect humans and mammals, but there are also free-living species.

A distinctive feature of representatives of the flatworm type is the three-layer structure (inner (endoderm), outer (ectoderm) and middle (mesoderm) layers), acquired in the process of evolution, as well as bilateral symmetry, differentiated tissues and organs.

Seven classes are included in the phylum of flatworms:

• Aspidogastra;
• Cestodoformes;
• Monogenea;
• Cilia;
• Tape;
• Gyrocotylides.

In appearance, worms from this class have a long, flat body with suckers on the front of the body for attachment.

Ciliated individuals have well-developed sensory organs, muscles, and have cilia for movement.

The body is leaf-shaped.

A special feature of the structure is the organ - a rosette that serves for attachment.

The main reasons for the increase in cases of invasion

In developed countries

In less developed countries

Many poor countries are strenuously taking all possible measures to combat unintentional infestations, while in developed countries cases of deliberate self-infection with flatworms by people following a diet to quickly lose weight are being recorded

Pests

North-western Europe, including the British Isles, is concerned about the current threat of the spread of planarians from New Zealand and the Australian worm Australoplana SANGUINEA. Triangulatus worms are believed to have arrived in Europe on plants brought in containers from botanical gardens.

Human uses of planaria

Philippines, Indonesia, Hawaiian Islands, New Guinea and Guam have successfully introduced two species of planarians, thereby controlling the growth of the population of the African snail species Achatina gigantea. Whether the spread of planaria was effective or not, the number of unwanted snails was significantly reduced. In this regard, there is an opinion that this method is more effective than other biological methods. But it is possible that planarians could subsequently pose a serious threat to their native snails.

Free-living species

The digestive system consists of the mouth, pharynx and intestines.

Both the consumption of food and the removal of waste products are carried out through the mouth, which is located on the ventral side at the front of the body.

Tapeworms do not have a digestive system.

Nervous system

Consists of cerebral ganglia, nerve trunks and nerves.
Nerves diverge from the trunks and the nerve ganglion, which cover all the tissues and organs of the worm.

Excretory system

Branching tubules serve for construction; at their ends there are cells in the form of asterisks, which are immersed in the parenchyma. They are called fiery or flickering. These tubules serve the worms to transfer trapped fluid waste from the parenchyma to the tubules via the cilia. The tubules extend to the surface of the worm, where they end in pores, through which waste is expelled from the body.

Reproductive system

Most flatworms are hermaphrodites. The organs of the reproductive system consist of ovaries and testes (simultaneously in one organism).

Sperm are contained in seminal fluid, which is produced by the testes - the male reproductive glands.

The process of fertilization occurs crosswise, which means that different individuals can exchange seminal fluid.

The skin-muscular sac includes organs: muscles and epithelium.

The epithelium has only one layer of cells. The surface can be covered with cilia, microvilli or chitinous hooks. Representatives of the class ciliated worms have cilia. Tapeworms, cestodes and other worms have microvilli and hooks.

Under the integumentary tissue there are immediately located muscles, which are divided into diagonal, longitudinal and annular. The body of worms is equipped with bundles of dorso-abdominal muscles. Such worms can stretch, taper, contract, bend, twist, and contract, thanks to a variety of muscles.

The sense organs consist of head tentacles that react to chemicals, photosensitive pigmented eyes, tactile cells located on the surface of the skin and the organ of balance.

Organs respiratory system none.

There is no circulatory system.

structural features

Body flatworms have clearly defined fabrics of all 4 main types, from which organs and organ systems are formed. The body shape is flattened in the dorso-ventral direction, has the appearance of a leaf, plate, ribbon, etc.

Symmetry bilateral (bilateral), appeared in the process of evolution as an adaptation to active image life. The body is already distinguished between the main and caudal ends, the dorsal and abdominal parts.

Germ layers - ectoderm, endoderm and mesoderm. During the process of embryonic development, the third germ layer is formed, which appears for the first time in flatworms. Mesoderm is the middle germinal layer characteristic of the embryos of multicellular three-layered animals.

Cavity there is no body, the spaces between the organs are filled with parenchyma. Parenchyma- loose connective tissue, performing various functions: storing nutrients, transporting them, removing metabolic products, maintaining body shape, etc.

Features of life processes

Support carried out thanks to parenchyma cells and the skin-muscle sac.

Movement provided smooth muscles, which are formed by circular, longitudinal and diagonal muscle fibers.

Transport of substances throughout the body occurs by diffusion. The circulatory system is absent in all flatworms.

Selection carried out with the participation excretory system. Appears for the first time in the process of evolution and is built according to the type of protonephridia. Protonephridia - excretory organs of some invertebrate animals, which consist of tubules, starting with stellate-shaped cells, from which cilia are directed into the lumen of the tubules. Metabolic products can accumulate in special parenchyma cells.

Regulation of functions implemented with the participation nervous system nodal (ganglionic) type, which is divided into central (CNS) then peripheral (PNS). The central nervous system consists of the head nerve ganglion and nerve trunks connected by annular septa. The PNS is represented by nerve processes and nerve endings. Only unconditioned reflexes are known in flatworms.

Reproduction sexual, which is carried out by the reproductive system of the female and male organs. In most flatworms reproductive system hermaphrodite. Hermaphrodites are organisms that have both female and male reproductive organs. Fertilization is internal, can be cross-fertilization or due to self-fertilization.

Regeneration well developed in free-living species.

Classification of flatworms lately suffered significant changes caused by the study of these individuals. Now the group of flatworms has 4 different classes.

Eyelash worms. The most primitive type of tapeworm and the only free-living one. In size it can reach up to half a meter in length.

Flukes or helminths that live in various human organs, mainly in the liver, lungs and intestines.

Nutrition and movement

Structure

The complex structure of the worm's muscles allows it to stretch and contract in size, as well as move and twist. The entire body of flatworms is filled with cells that form a loose mass. This connection of cells is called parenchyma. This is where the excretory system, digestive organs and genitals are located. The excretory system has protonephridia, which remove all unprocessed food from the body. The secretion can pass through cells or through excretory tubules.

Although digestion occurs in all classes, the organ system is present in only a few representatives. Other species receive nutrition through the integument of the body, so the system itself may be absent. The digestive system has a blind ending.

Not all types of these worms have sense organs. In free-living representatives, the sense organs are represented by vision. Like other primary cavity organisms, tapeworm flatworms do not have a circulatory system. Also, flatworms have no sexual division; all representatives are hermaphrodites. The development of helminths occurs in a direct way.