Metagenesis

METAGENESIS (from Greek meta- after and genesis- origin, birth), a complex method of reproduction consisting in the fact that the complete life cycle of an animal proceeds with a change in methods of reproduction, usually in two generations succeeding one another. The malarial plasmodium reproduces in the human blood asexually (by schizogony), while in the body of Anopheles it undergoes sexual reproduction. Hydrozoans, by budding (asexual reproduction), form a colony in which, besides vegetative individuals (polyps), medusae are formed, which separate and float freely. In the medusae, sex cells develop, which in subsequent division (sexual reproduction) give rise to larvae that, by budding again, form the colony form, and so on. M. is widespread among many parasitic worms; for example, the sexual form of the echinococcus (Taenia echinococcus) - a tapeworm - produces numerous eggs, from which bladder worms are formed; these latter (asexual reproduction) produce secondary bladders and brood capsules with numerous heads. In M., the sexual generation alternates with the asexual and vice versa; hence the term "alternation of generations." In parasites that reproduce metagenetically, the change of generations is associated with a change of hosts. METAGONIMOSIS (metagonimosis), a helminthic disease of the human intestine and fish-eating mammals, caused by the trematode Metagonimus yokogawai Katsurada 1912, belonging to the family Heterophyidae. Synonyms of the parasite: M. romanicus Ciurea 1915, M. parvus Ciurea 1915, Loossia romanica Ciurea 1915, Loossia parva Ciurea 1915, Loossia dobrodjiensis Ciurea. - The body is pear-shaped, 1.0 to 2.5 mm in length, with a maximum width in the posterior half of the body of 0.4-0.8 mm. The cuticle is covered with spines, more strongly expressed in the anterior half of the body. The oral sucker reaches 0.07-0.09 mm in diameter, pharynx 0.05-0.06x0.045-0.055 mm; there is a prepharynx; postpharynx is elongated; the intestinal trunks branching from it extend to the lateral edges of the parasite and run parallel to them to the posterior end of the body. The genital sucker is displaced from the midline of the body to the right side of the parasite, adjacent to the inner edge of the right intestine, approximately at the level of the middle of the body length. This sucker is oval-shaped, with its long axis directed at an acute angle to the axis of the parasite. The length of this structure is 0.12-0.145 mm with a width of 0.075-0.10 mm. Round-oval testes reach 0.20-0.30 mm in diameter, are located obliquely to each other and occupy the posterior part of the body, adjacent with their edges to the terminal portions of the intestine. Between the testes is a V-shaped excretory bladder. In front of the anterior testis, usually medially, is located the receptaculum seminis, to the anterior or lateral edge of which is adjacent the spherical ovary, located along the midline of the body. The vitellaria are located in the posterior half of the body, adjacent to the lateral edges, consist of a relatively small number of large follicles (about 10-12 on each side) and cover the intestinal trunks. The terminal male apparatus consists of a powerful vesicula seminalis, resembling a 2-3-kneed tube, the fragments of which are located at an acute angle to each other. The uterus is located in the posterior half of the body, filling its convolutions all the spaces between the genital elements. The terminal part of the uterus, directed toward the genital sucker, passes forward from the terminal part of the vesiculae seminalis. Mature eggs of a light grayish-yellow shade reach 0.026-0.032 mm in length with a width of 0.015-0.017 mm. The pole of the egg where the operculum is located is somewhat narrower than the opposite more rounded end, on which a small pimple-like thickening of the shell is observed. The biology is characterized by the presence of 2 intermediate hosts: the first is the mollusk Melania libertina, the second is salmon fish (Plecoglossus altivelis) and some carp fish (crucian carp, dace, etc.). - The parasite is widespread in Japan, Korea, China. In the USSR, the parasite was recorded in the Far East in the lower Amur and along the tributaries of the Ussuri among the northern peoples - Gilyaks, Golds, Ulchs and Udegeys, who consume completely raw fish. According to the 1929 survey data (60th Union Helminthological Expedition), 13.6% of the indigenous population of the Far Eastern region is infected with this parasite, located in the small intestine. - Clinically, M. manifests itself only with intense invasion and is characterized by symptoms of dyspepsia, diarrhea, anemia. - Therapy: administration of laxative salt; after 8 hours - thymol in a dose of 2.0 three times in succession every 2 hours; after 3 hours after the last dose - laxative salt. On the second day, a new administration of thymol at 2.0 three times with two-hour intervals. After 2 hours - laxative salt. Thus, for the entire course of treatment, 12.0 of thymol is used (Skrjabin, Podjapolskaja and Shihobalova). - Prevention consists in consuming well-cooked fish and protecting water bodies from contamination with feces. It would be advisable to carry out mass deworming of metagonimosis-infected dogs and cats, however, this measure is currently not practically feasible.

K. Skrjabin. METAZOA, a name denoting the entire aggregate of multicellular animals in opposition to unicellular, Protozoa. M. are characterized by the specialization of cells that make up their body. Submitting to the principle of physiological division of labor, the cells of the body of M. differentiate in various directions, as a result of which several types of cells are obtained, differing from each other both in structure and in function: nerve, epithelial, muscle, sexual, etc. To the number of M. belong 7 types of animals: coelenterates, sponges, worms, mollusks, arthropods, echinoderms and chordates. Some zoologists isolate sponges from typical M. due to the absence of a clearly differentiated internal germ layer associated with the digestive function and the similarity of sponges to some colonial protozoa.