RENAL SYSTEM OF FISHES. 283 



In most osseous fishes the kidneys are long and narrow, and ex- 

 tend through the whole or a great part of the dorsal region of the 

 abdomen, firmly attached to the vertebral column ; they are usually 

 broadest and thickest anteriorly, where they sometimes present a 

 lobulated surface ; they contract, approximate, and frequently blend 

 together as they extend backwards (prep. 1 177. Cyclopterus) ; some- 

 times penetrating the haemal canal in the tail. In the Gymnotus the 

 kidneys are distinct and thickest at their posterior ends, as they are 

 in the Gurnard i^jig. 72. K) and in most Sharks. The kidneys have 

 not a well-defined capsule in osseous fishes, but their ventral sur- 

 face is immediately covered by an aponeurotic membrane, against 

 which the peritoneum, and the air-bladder when present, are applied. 

 The renal tissue is soft and spongy, firmest at the fore-part of the 

 gland ; usually of a reddish -brown colour ; sometimes soaked, as it 

 were, with dai'k pigment (as in Lepidosiren, xxxiii. p. 349.). It is sup- 

 plied by numerous small arteries from the abdominal aorta *, which 

 form Malpighian corpuscles ; but these are fewer in number and 

 less complex than in the true kidneys of higher Vertebrates. The 

 primary branches of the tubuli uriniferi, given off from the long 

 ureter, are extremely numerous ; their divisions in tlie renal sub- 

 stance are comparatively few ; they are in most fishes convoluted and 

 of equal diameter, extending through the whole renal substance, 

 which shows no distinction of cortical and medullary parts, and has 

 no mammilljE : they are lined by a ciliated epithelium. Sometimes 

 a single common ureter quits the coalesced hinder ends of the kidneys, 

 as in the Pike, and terminates in a urinary bladder. More frequently 

 the essentially duplex nature of the kidneys is manifested by the 

 emei'gence of two ureters from the ventral surface of their posterior 

 ends when these have coalesced : in some fishes these unite together 

 after quitting the kidneys, and terminate by a common gradually 

 widening canal in the urinary bladder. Sometimes they enter the 

 urinary bladder separately, as in tlie "Wolf-fisli, where they both 

 terminate on its left side, half-an-inch above the cervix : rarely are 

 any smaller accessory ureters seen, as e. g. in the Stickleback, to 

 terminate also, separately, in the bladder. This, in aquatic animals 

 apparently needless, receptacle of a fluid excretion is, nevertheless, 

 rarely absent in osseous fishes ; the Pilchard, the Herring, and the 

 Loach are among the few instances where it is not developed. In 

 the Loach a very short, in the Herring a long, common ureter ter- 

 minates behind the anus. In the Gymnotus the common ureter is so 

 wide as to serve as a receptacle, and it is directed forwards to reach 



" Hunter, \u. t. ii. p. 112. 



