. Elementary text-book of zoology, general part and special part: protozoa to insecta. Animals. thematoba. 323 cases {Tristomum coccineum) transverse rows of bristles are found. Paired eyes are frequently pi'esent. In some species the elongated body presents a kind of external segmentation. They are for the most part ectoparasitic, to a certain extent like the IlirucUnea, and they develop directly without alternation of generations from eggs which are usually hatched in the locality inhabited by the mother. Sometimes the development is a metamorphosis {Fobjstomum), and the young larvte live in
. Elementary text-book of zoology, general part and special part: protozoa to insecta. Animals. thematoba. 323 cases {Tristomum coccineum) transverse rows of bristles are found. Paired eyes are frequently pi'esent. In some species the elongated body presents a kind of external segmentation. They are for the most part ectoparasitic, to a certain extent like the IlirucUnea, and they develop directly without alternation of generations from eggs which are usually hatched in the locality inhabited by the mother. Sometimes the development is a metamorphosis {Fobjstomum), and the young larvte live in another place. The development of Pohjstomum integerri- mum from the bladder of the frog is the best known, owing to the researches of E. Zeller (figs. 258, 259). The production of eggs begins in the spring, when the frog awakes from hibernation and proceeds to pair. It lasts from three to four weeks. It â ^⢠is easy then to observe the Polysto- mea in the process of reciprocal copulation. When the eggs are being laid, the parasite forces the anterior end of the body with the genital Fig. inte- ° fferrimum (after E. Zeller). opening through the mouth or the bladder o, mouth; Go, genital nearly as far as the anus. The development opening; D, intestine; â¢' _ 'â ly, copulatory opening of the embryo takes place in water and occu- (lateral pads); D^, yolk pies a period of many weeks, so that the orovaJ^â¢'zr hooTs!^''' young larvfB are only hatched when the tad- poles have already acquired internal gills. The larvae resemble GyrodactTjlus, and possess four eyes, a pharynx and alimentary canal, as well as a posterior disc (for attachment), which is surrounded by sixteen hooks. They possess five transverse rows of cilia; three are ventral and anterior, two dorsal and posterior. There is also a ciliated cell upon the anterior extremity. The larvaa now migrate. Fig. 259.âEgg with embryo{o),and hatched larva (i) of PoJystomum integerrimum ; Dk, operculum (af
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectanimals, bookyear1892
