VERTEBKATA. 313 



may be recurved spines, as in the Pike ; flat and triangu- 

 lar, with serrated edges, in the Shark ; or flat and tessel- 

 lated in the Kay. They feed principally on animal mat- 

 ter. The digestive tract is relatively shorter than in other 

 Vertebrates.'" 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, which 

 are covered by the operculum, and the water escapes from 

 an opening behind this. In Sharks there is no operculum, 



Fig. 28B.— Siilnion (S»fmo aaker). Both hemispheres. 



and each gill opens separately. The brain consists of sev- 

 eral ganglia placed one behind the other, and occupies but 

 a small part of the cranial cavity. Its average weight to 

 the rest of the body may be as low as 1 to 3000. The 

 eggs of bony Fishes are naked and multitudinous, some- 

 times numbering millions in a single spawn ; those of the 

 Sharks are few, and protected by a horny shell (Fig. 164). 

 There are about thirteen thousand species of Fishes, of 

 which over two thirds are Teleostei. There are two sub- 

 classes of Pisces. 



