604 



COMPARATIVE ANATOMY. 



Petroinyzontes it is first connected with the oue on the opposite 

 side, to form an unpaired and wider portion. We do not yet know 

 its relation to the metameric ciliated infundibula. 



In the Selachii the primitive arrangement is limited to the early 

 stages of development. The primitive kidney extends along the 

 dorsal wall of the coelom, and is made up of separate canaliculi, 

 which commence by ciliated iufundibula (Fig. 314, i), which open 



into the abdominal cavity. Each canal, 

 after having broken up so as to en- 

 close a glomerulus (m), is continued 

 on into the archinephric duct. These 

 canals increase in length, so that each 

 of them forms a coiled lobule (r) ; each 

 kidney, therefore, is composed of a 

 series of these coils, which are col- 

 lected together into the archinephric 

 duct (u). This duct opens into the 

 cloaca. Changes may occur in the 

 glandular portion of these kidneys, as 

 well as in their efferent ducts. The 

 anterior portion, which is made up of 

 a number of lobules, does not undergo 

 any very great development, as does 

 part of the hinder portion. This, 

 which is made up of a varying, but 

 large number of primitive lobules 

 (13-14 in Acanthias), is converted 

 into a larger organ, the canaliculi in 

 which may be seen to increase in 

 number by budding off new ones. 

 This portion retains its renal function, 

 while the anterior part is atrophied, 

 and, in the male, enters into con- 

 nection with the generative gland. 

 The ciliated funnels (nephrostomata) 

 are retained in some Sharks only ; they disappear in all the Rays, 

 and many of the Sharks. Where they are retained they are reduced 

 in number. 



Of the changes which obtain in the primary archinephric duct 

 the most important is its division into two parts. This commences 

 at its anterior end, and extends backwards, so that there come to be 

 two canals. One commences at the anterior abdominal orifice of 

 the primary duct, and has no further relations to the kidney. This 

 is the Mullerian duct. The other canal retains its connection 

 with the primitive kidney, and forms the secondary archinephric 

 d dot. But even this portion may undergo certain changes, inasmuch 

 as in the male it is converted into the seminal duct. The efferent 

 ducts from the posterior portion of the kidney are then collected 

 into a common ureter, which opens into the sinus urogenitalis, into 



Fig. 344. Portion of the kidney 

 of an Embryo of Acanthias (dia- 

 gram), i Ciliated funnel, m Mal- 

 pighian body, r Renal lobules. 

 u Archinephric duct. 



