FISHES. 349 



ters begin by numerous roots, and run along the under sur- 

 face of the kidney. They terminate either in a vesica uri- 

 naria, or a cloaca ^ or unite together to form a dilatation, 

 which supplies the place of a bladder of urine. 



In the chondropterygii, the ureters terminate in the clo- 

 aca, but in the other cartilaginous fishes the bladder of 

 urine is present, although very small and thin in its coats. 

 The urethra in most fishes is short, and commonly opens 

 behind the anus by an orifice, which also gives issue to the 

 sexual evacuations. Renal glands are wanting in this class. 



The Reproduction of fishes is a subject involved in 

 great obscurity. The element in which they reside con- 

 ceals from us the actions which they perform, so that we are 

 unable to point out, with certainty, the uses of the different 

 organs, or the functions which they exercise. Even in the 

 days of Aristotle, the difference in the mode of repro- 

 duction between the cartilaginous and the osseous fishes 

 had been observed ; and although many accurate observa- 

 tions have been made by modern zootomists, much still re- 

 mains to be done, both in the field of observation and dis- 

 section. 



In reference to the reproductive system, fishes may be 

 divided into two classes. Thus, some have the sexes dis- 

 tinct, while in others they are united. Those with the sexes 

 distinct, may be subdivided into such vas hdve the impreg- 

 nation External or Internal. 



Fishes with the sexes distinct, and with external impreg-- 

 nation. — The fishes included under this division, are by far 

 the most numerous. They have all free branchiae. Some 

 of them possess a cartilaginous skeleton, while others belong 

 to the division termed Osseous. In all of them, the egg is 

 impregnated externally, and arrives at maturity without the 

 aid of the mother. 



