Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0101clau Year: 1884 TREMATODA. 321 ,/faviim and mutabile), which it carries about until it enters the first host (fig. 256, b). In other cases the course of development is sim- plified by the omission of the second intermediate host, viz., that which contains the encysted immature Distomum (Cercaria macro- cerca of Distomum cygnoides, also Leucochloridium in the tentacles of Helix s/iccinea). (1) Sub-order : Distomea. Trematodes with at most two suckers, without hooks. They develop with a complicated altern
Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0101clau Year: 1884 TREMATODA. 321 ,/faviim and mutabile), which it carries about until it enters the first host (fig. 256, b). In other cases the course of development is sim- plified by the omission of the second intermediate host, viz., that which contains the encysted immature Distomum (Cercaria macro- cerca of Distomum cygnoides, also Leucochloridium in the tentacles of Helix s/iccinea). (1) Sub-order : Distomea. Trematodes with at most two suckers, without hooks. They develop with a complicated alternation of generations. The asexual individuals and the larvae live principally in Mollusca, the sexually mature animals in the alimentary canal of Vertebrates. The sexes are completely separated in Distomum Ita'matobium (from the veins of man); individuals of the two sexes being united in pairs (fig. 257). Dimorphic forms are found in certain species of the genera Monostomum and Distomum in connection with the division of labour of the sexual functions; one individual develops only male sexual organs, and the other only female, the former producing spermatozoa and the latter ova. The rudiment of the functionless generative gland undergoes in these cases a more or less complete degeneration. Such Distomea are morphologically hermaphrodite, but practically of separate sexes. The complete biology and developmental FIG. 257—JD'sfomum hiemato- history is unfortunately only satisfactorily known for a few species which can be fol- lowed through all the stages of development. Hum. Male and female, the latter being in the canalis gyntecophorus of the former. S, sucker. Fam. Monostomidae. Of an oval, elongated, more or less rounded form, with only one sucker, which surrounds the mouth. Monostomum Zeder. Sucker surrounding the mouth ; pharynx powerful. Sexual openings but slightly removed from the anterior end. J/. nn/titl>/l<' Zeder, in the body cavity and in the orbit of vario
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