BOO: COMPARATIVE ZOOLOGY. 
free for the development of muscles which are to move 
the tail—the instrument of locomotion. The biconcave 
vertebra, with intervening cavities filled with elastic gel- 
atine, are designed for rapid and versatile movements. 
The body is either naked, as in the Eel, or covered with 
polished, overlapping scales, as in the Perch. Rarely, as 
in the Sturgeon, it is defended by bony plates, or by mi- 
nute, hard spines, as in the Shark. 
The vertical fins (dorsal, anal, and caudal) are peculiar 
to Fishes. ‘The dorsal vary in number, from one, as in 
the Herring, to three, as in the Cod; and it may be soft, 
as in the Trout, or spiny, as in the Perch. If the dorsals 
Fig. 273.—Blue-fish (Temnodon saltator). All seas. 
be cut off, the Fish reels to and fro. The caudal may be 
homocereal, as in ordinary species; or heterocercal, as in 
Sharks. In ancient heterocercal Fishes, the tail was fre- 
quently vertebrated. The pectoral and ventral fins stand 
for the fore and hind limbs of other Vertebrates. As the 
specific gravity of the body is greater than that of the 
water, most Fishes are provided with an_ air - bladder, 
which is an outgrowth from the cesophagus. This is ab- 
sent in such as grovel at the bottom, as the Rays, and in 
those, like the Sharks, endowed with compensating mus- 
cular power. 
Fishes have no prehensile organ besides the mouth. 
