. Elementary text-book of zoology, general part and special part: protozoa to insecta. Animals. TEEMATODA. 325 form an X-sbaped double animal, tbe posterior ends of which arc provided with two large suckers divided into four pits. In the young state they live solitarily as Blporpa ; they then possess a ventral sucker and a dorsal papilla (2G0 a, G and Z'). In the double animals the formation of ova is confined to a defimte period of the year, usually the spring. The eggs are laid singly after the forma- tion of the thread by which they arc attached, and two weeks later the embryo (fig. 261, *)
. Elementary text-book of zoology, general part and special part: protozoa to insecta. Animals. TEEMATODA. 325 form an X-sbaped double animal, tbe posterior ends of which arc provided with two large suckers divided into four pits. In the young state they live solitarily as Blporpa ; they then possess a ventral sucker and a dorsal papilla (2G0 a, G and Z'). In the double animals the formation of ova is confined to a defimte period of the year, usually the spring. The eggs are laid singly after the forma- tion of the thread by which they arc attached, and two weeks later the embryo (fig. 261, *), which only differs from Dipiu'pa in the possession of two eye- spots and a ciliated apparatus upon the sides and on the posterior extremity of the body, is hatched. When an oppor- tunity of fixing itself on the gills of a fresh-water fish occurs, the young animal loses its cilia and becomes a Biporpa, which possesses, besides the characteristic apparatus for attachment, the alimentary canal, and the two excretory canals with their openings at the anterior part of the body (at the level of the pharynx), and sucks the branchial blood. The junction of the two Dijwrpa soon follows ; and this does not take place, as was formerly believed, by the fusion of the two ventral suckers, but in such a manner that the ventral siicker of each animal affixes itself to the dorsal papilla of the other, and fuses with it (fig. 260, V). B. imracloxum v. Kordm,, on the gills of many fresh- water fish. Fam, Gyrodactylidae. Very small Tre- matodes with large terminal caudal disc and powerful hooks. They are viviparous, producing a single young one (first gene- ration) at a time, -within which, while still in the body of the parent, another young one (second generation) may be present, and in this yet another (third generation). V. Siebold believed that he had observed a young animal developing from a germ cell of Gyrodactylus, and that this became pregnant during its development. He regarded the Gy
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectanimals, bookyear1892
