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 the collecting tube, and functions as the ureter. 

 In the Myxinoidea and Petromyzontes, the kidneys, which are set 

 along the posterior third of the ccelom, are larger, but the urinary 

 canaliculi have exactly the same relations. In both forms the 

 ureter takes a lateral course to the abdominal pore ; but in the 



Fig. 313. A Portion of the 

 kidney of Bdellostoma. 

 a Urinary duct, b Urinary 

 canaliculi. c Terminal capsule. 

 B Portion of the same more 

 highly magnified, a c As before. 

 In c there is a glomerulus. 

 d Afferent, e Efferent artery 

 (after J. Midler). 



