414 ZOOLOGY. 



tinct. The ovaries are large bodies, either discharging the 

 eggs directly, as in the eel, salmon, and trout, into the body- 

 cavitv, thence passing along a fold of the peritoneum out of 

 a minute opening situated directly behind the vent, or, as in 

 most bonv fishes, there is a duet leading from each ovary to 

 the common outlet. In the sharks and skates (Elasmo- 

 branchs) the ovary is single, and the oviducts unite behind 

 to serve as a uterus in such sharks as are viviparous ; or the 

 same parts secrete a shell in the egg-laying sharks {Scyllium) 

 and skates. 



The reproductive glands of most fishes are. except in the 

 breeding season, so much alike, that it is difficult to distin- 

 guish them except by a microscopic examination. In the 

 breeding season the ovaries of the cod, perch, and smelt are 

 very large and yellowish, while the testes are small and wliite. 

 Fishes, like some Amphibians and many invertebrates, may 

 be aljle to perform the reproductive functions before they are 

 fully mature-; in fact, some fishes continue to grow as long 

 as they live. 



The fishes are not a homogeneous or '■closed," i. e., well- 

 circumscribed, t}']ie, as the birds and mammals, for the form 

 of the Ijody is liable to great variation, the differences be- 

 tween the subdivisions or orders, families and genera being 

 much greater than m birds and mammals. 



The class is divided into three subclasses, viz. : the Elas- 

 7)!o5/'rt»e/n't (sharks and rays), the Ganoidei (sturgeons, gar- 

 pikes, etc.). and the Teleoslei, or bony fishes. The classifi- 

 cation we adopt is that of Professor Gill. 



Subclass 1. HHasmobranchii {St'hicJtians, or SJiarks and 

 llayx). — These are the must generalized as well as among the 

 oldest of all fishes. In some respects they stand alcove the bony 

 fishes, with some features anticipating the Amphibians, while 

 in their cartilaginous skeleton, their numerous gill-openings, 

 and their general appearance they are scarcely higher than the 

 embryos of the bony fishes. It would seem as if a shark were 

 an embryo fish, which had been luirried by nature into exist- 

 ence with some jiarts more perfect than others, in order to 

 serve in the Upper Silurian and Devonian times as destructive 



