VERTEBRATA. 301 
Both jaws are alike movable. The teeth are numerous, 
and are generally recurved spines, as in the Pike; flat and 
triangular, with serrated edges, in the Shark; and tessel- 
lated in the Ray. They feed principally on animal mat- 
ter. The digestive tract is relatively shorter than in oth- 
er Vertebrates.'” With one exception (the Amphioxus), 
the blood is red, and the heart has rarely more than two 
cavities, an auricle and a ventricle, both on the venous 
side. Ordinary Fishes have four gills, the water escaping 
by one external aperture, or “ gill-slit;” but in the Sharks 
Fig. 274.—Salmon (Salmo salar). Both hemispheres. 
there is a separate opening for each gill. The brain con- 
sists of several ganglia placed one behind the other, and 
occupies but a small part of the cranial cavity. Its aver- 
age weight to the rest of the body is as low as 1 to 3000. 
The eggs of bony Fishes are naked, and multitudinous, 
sometimes numbering millions in a single spawn; those of 
the Sharks are few, and protected by a horny shell. 
There are six orders of Fishes: 
1. Pharyngobranchs, represented by a single species, 
