414 ZOOLOGY. 



tinct. The ovaries are large bodies, either discharging the 

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

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

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

 most bony fishes, there is a duct 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, Avhile the testes are small and white. 

 Fishes, like some Amphibians and many invertebrates, may 

 be able 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, type, as the birds and mammals, for the form 

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

 tween the subdivisions or orders, families and genera being 

 much greater than in birds and mammals. 



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

 ')iu>br<tnr/iii' (sharks and rays), the (litnoidei (sturgeons, gar- 

 pikes, etc.), and the Tt'?/'/i*/t'i. or bony fishes. The classifi- 

 cation we adopt is that of Professor Gill. 



Subclass 1. Elasmobranchii (Selachians, or Sharks and 

 J?t/x). These are the most generalized as well as among the 

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

 fishes, with sonic features anticipating the Amphibians, while 

 in their cartilaginous skeleton, their numerous gill-openings, 

 and their general appearance thev are scarcelv higher than the 



v t/ O 



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

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

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

 serve in the Upper Silurian and Devonian times as destructive 



