FLAT WORMS (Plathelminthes; synonym: Platodes, Platdaria, Plathalmia) — the type of worms combining eight classes (Turbellaris, Temnocephala, Udonellida, Me-sozoa, Trematoda, Monogenea, Gyro-cotylida, Cestoda).
In total is apprx. 7300 types of P. of h. A number of types of P. of h — inhabitants of sea and fresh reservoirs; many types adapted to a parasitic way of life. At the person representatives of the classes Trematoda parasitize generally (see. Trematodoza ) and Cestoda (see. Tsestodoza ). Ciliary hearts (Turbellaria) and nemertina are, as a rule, free living organisms.
The item of h — multicellular, bilateral and symmetric animals. Their body is flattened in the dorsoventral direction, usually has foliate or pear-shaped (trematodes), taenioid (cestodes or tape-worms), is more rare — a cylindrical form. Length of a body fluctuates from 0,1 mm (trematodes this. Microphallidae) to 15 m (a cestode of Diphyllobothrium latum).
At trematodes the body is not dismembered. At cestodes the body is divided into several departments: the head (scolex) bearing bodies of an attachment (a sucker, kryuchya, prisasyvatelny cracks — bothriums), the neck which is region of growth; the neck passes into actually body (strobile) consisting usually of joints (proglottids) which number varies from three to several thousand.
The body wall of P. of h represents the skin and muscular bag consisting of a surface layer (tegument) and several muscular layers (at parasitic forms), or from the single-layer epithelium covered with cilia and one or several layers of muscles (at free living). The basal membrane separates a tegument from three layers of smooth muscles — ring, longitudinal and diagonal. The space between P.'s bodies of h is filled with a parenchyma of a mesodermal origin. It performs basic function and plays an important role in processes of exchange.
The nervous system of P. of h is formed by a pair head ganglion, or a nervous ring, and nervous trunks departing from it connected by crossing points.
The alimentary system consists of an ectodermal front gut, or a throat, and an entodermalny average gut. P. have a p. of no back gut and an anus. At the representatives of the class Cestoda parasitizing, as a rule, in intestines, the alimentary system is absent. Food of cestodes occurs through covers at the expense of the food of the owner which is previously split went to it. - kish. path.
Eliminative organs of P. of h are constructed as protonephridiums. They consist of the star-shaped cells scattered in a parenchyma having a bunch of cilia from which the branched tubules combined in one (at trematodes) or in two depart (at cestodes) excretory the channel terminating in secretory openings (time).
In the majority of P. of h — gerkhmafrodita. Their reproductive system is arranged variously and difficult.
The difficult life cycle connected with alternation and change of owners is characteristic of many P. of h. So, it is necessary for development of the trematodes parasitizing in intestines or other internals of the person and animals that egg after escaping of an organism ha-zyaina got to water; in water leaves it covered with cilia of the world-tsidy. Further development of the world-tsidiya takes place in a body of the intermediate owner (most often a mollusk) where it turns into a shapeless bag — to a sporocyst. In a sporocyst from formative cells parthenogenetic (see. Parthenogenesis ) mobile rediya are formed. Further from separate formative cells of a rediya cercariae form, to-rye have a long tail, suckers, couple of eyes, intestines, sometimes a stylet. Cercariae, having left a body of a mollusk, actively float in water by means of a tail. At the majority of trematodes the development cycle is connected with two intermediate owners, and the second are larvae of water animals (insects, fishes etc.). In this case cercariae by means of an acute stylet destroy covers of the second intermediate owner, are implemented in his body and intsistirutsya (become covered by a cover), turning into the following stage — metacercarias. In an organism of the final owner of a metacercaria turn into an adult parasite.
Two larval stages — an oncosphere (a shestikryuchny germ) and the Finn are characteristic of development of cestodes. A life cycle of cestodes can be connected with one (Taenia solium) or two (Diphyllobothrium latum) intermediate owners.
Many trematodes and cestodes are causative agents of a serious illness of the person. Infection of the person occurs at consumption of crude fish (the Siberian fluke, a wide tape-worm, etc.) or insufficiently processed meat (the armed tapeworm, an unaided tapeworm, etc.), at a proglatyvaniye of eggs (see. Echinococcosis ), and also as a result of active penetration of larvae through skin (see. Shistosomatoza ).
Fight against parasitic P. of h includes system of preventive actions on a dignity. to education of the population, observance of rules of personal hygiene, sanitary and veterinary control on slaughters, meat-processing plants, the markets, heat treatment of fish before her use in food etc. Release of the person from parasitic P. of h (deworming) happens by means of various antihelminthic drugs, and sometimes (at an echinococcosis, etc.) and a surgical way (see. Deworming ).
Bibliography: Dogel V. A. Zoology of invertebrates, page 135, M., 1975; Pavlovsky E. N. Guide to parasitology of the person, t. 1, page 210, M. — JI., 1946; Schultz R. S. and in about z d e in E. V. Bases of the general helminthology, t. 1, page 85, M., 1970.
Yu. K. Bogoyavlensky.