Elements of comparative anatomy (1878) Elements of comparative anatomy elementsofcompar00gege Year: 1878 EXCRETORY ORGANS OF VERTEBRATA. 603 surface of the metameres themselves (§ 145), and not into a longitu- dinal canal. This canal is the part which, by being the first part to appear in the Vertebrata, defines the type of the whole apparatus. But, just as in a large number of Invertebrata the excretory organs partly lose their function, and serve as efferent ducts for the generative products, so too among the Vertebrata do we meet with a process of this kind, by which great changes are effe
Elements of comparative anatomy (1878) Elements of comparative anatomy elementsofcompar00gege Year: 1878 EXCRETORY ORGANS OF VERTEBRATA. 603 surface of the metameres themselves (§ 145), and not into a longitu- dinal canal. This canal is the part which, by being the first part to appear in the Vertebrata, defines the type of the whole apparatus. But, just as in a large number of Invertebrata the excretory organs partly lose their function, and serve as efferent ducts for the generative products, so too among the Vertebrata do we meet with a process of this kind, by which great changes are effected in the primitive excretory apparatus. It loses, and that often very early in life, its primitive arrangement. And when this is not seen in the embryo, it must be regarded as due to new, acquired, relations. 1 § 448. In the Cyclostomata, Teleostei, and Amphibia, a special portion of the primi- tive kidney appears at the most anterior end of the archinephric duct, which de- serves especial mention, as it does not only appear earlier than the rest of the primitive kidney, but is' generally sepa- rated by some distance from it. This portion is made up of a small number of canaliculi, which commence by ciliated infundibula, and are generally set in a coil. There may be only one canaliculus. A Malpighian body may sometimes be observed on the canaliculi. In the Amphibia this pronephron undergoes atrophy, while in the Amniota it does not seem to be even rudimentarily de- veloped. It persists, however, in the Cyclostomata, where it is provided with a tuft of ciliated infundibula, which project into the abdominal cavity. Among the Cyclostomata the primi- tive kidney is seen at its simplest in Bdellostoma. An elongated canal (Fig. 343, A B, a) gives off short transverse canaliculi at various points (b); the blind end of which (c) is constricted off, and encloses a glomerulus (B). The transverse canaliculi form the secretory apparatus (urinary canaliculi); the archi- nephric duct is
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