12 FISHES OF THE VICINITY OF NEW YORK 



of the gills; after passing through the minute tubes and renewing its 

 supply of oxygen, it leaves the gills on the upper side and is collected 

 into a central pipe called the dorsal aorta, from which in turn it is distri- 

 buted to the body, where it is again deprived of its oxygen and sent back 

 to the gills. In most fishes the circulation of the blood is comparatively 

 sluggish and the body temperature is low, but in some very active types, 

 such as the tunny, the process of oxidation and the circulation of the 

 blood are quickened and the body temperature is greatly raised. 



The jaws of sharks, which are still in a primitive condition, consist 

 of two pairs of bars underneath the head, one pair forming the upper 

 jaw and another the lower jaw. These bars consist of gristle, or cartilage, 

 stiffened by the deposition of calcium carbonate 1 . The jaws of sharks 

 agree with the gill bars behind them in their minute structure, in their 

 relations with the throat cavity and nerves and to a certain extent even 

 in general appearance. It is highly probable that the jaws originally 

 served to support a pair of gills, that they were in fact true gill-arches 

 and shared in the opening and closing movements of the gill-arch frame- 

 work; in the course of long ages, as the animals came to pursue larger 

 prey, the gill-arch-jaws became larger, and finally were used exclusively 

 for snapping at the prey, which was also held fast by the thorny skin 

 around the mouth. The jaw muscles of sharks are very simple, being 

 wrapped around the back part of the upper and lower jaws and serving 

 to squeeze the jaws together. These jaw muscles are comparable with 

 the muscles behind them that work the gill-arches. 



The jaws of ordinary sharks are attached to the under side of the 

 skull by ligament and to the back of the skull by a cartilaginous bar, 

 called the hyomandibular, which represents the upper part of an old 

 gill-arch. Between the hyomandibular and the skull and behind the 

 eye there is a small opening, called the spiracle, which was formerly a 

 gill-cleft but is greatly reduced or even absent in most fishes; in the 

 skates, however, it serves chiefly as an intake valve for water for the 

 gill chamber. 



The jaws of typical fishes such as the bass are much more com- 

 plicated than those of sharks and have been evolved from the more 

 primitive conditions described above. The primary, or cartilaginous, 

 upper jaws become saturated with bone cells, which deposit phosphate 

 of lime and thus form true bony upper jaws; meanwhile the skin of the 

 lips has produced bony plates which form a secondary, or false, upper 



'There are no bone cells in any part of the skeleton of sharks and Rays. Hence the name 

 "Cartilage Fishes" (Chondrichthyes) often applied to this group. 



