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 
4 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 (Hlasmo- 
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 (Scylliwm) 
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 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,” 7. 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 £las- 
mobranchti (sharks and rays), the Ganoidei (sturgeons, gar- 
pikes, etc.), and the Teleostei, or bony fishes. The classifi- 
cation we adopt is that of Professor Gill. 
Subclass 1. Elasmobranchii*(Selachians, or Sharks and 
Rays).—These are the most generalized as well as among the 
oldest of all fishes. In some respects they stand above 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 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 
* See Miller and Henle, Hasse, Balfour, Wyman, Garman, etc. 
