520 



PISCES FISHES. 



The aorta, while in the abdomen, runs beneath the spine, and 

 gives arteries to the viscera in the usual manner ; but at the com- 

 mencement of the tail it becomes enclosed in the inferior vertebral 

 arches, by which it is defended to its termination. 



(555.) There is yet another set of organs, which, as we ascend 

 from inferior to higher forms of animal life, we encounter for the 

 first time in the class before us ; an apparatus for elaborating the 

 urinary secretion, which is peculiar to the Vertebrate classes. 



The kidneys in fishes are very voluminous : they are situated on 

 each side of the mesial line, immediately beneath the bodies of the 

 vertebrae ; and extend along the whole length of the abdomen, not 

 unfrequently reaching to the base of the skull, where their anterior 

 portion (Jig. 27, e) lies above the branchial apparatus. The ure- 

 ters (Jig. 27, f) generally terminate in a kind of bladder-like di- 

 latation, the orifice of which is found behind that of the vulva (s). 



Examined minutely, the substance of the kidney is found to be 

 entirely composed of microscopic tubules, which terminate in the 

 ureters : these uriniferous tubes are variously contorted, but of 

 equable diameter throughout ; and they end towards the periphery 

 of the kidney by blind extremities. 



(556.) The skin of these aquatic animals is perpetually lubri- 

 cated by an abundant mucous secretion furnished by muciparous 

 follicles, or secreted in long tubular organs placed beneath the 

 skin. In the Skate the vessels last mentioned are remarkably large, 

 and their distribution very extensive. 



Fig. 229. 



