148 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



rest of the body may be as low as I 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 (Fig. 360). 

 There are about thirteen thousand species of fishes, 

 of which over two thirds are Teleostomi. There are 

 three principal subclasses of Pisces. 



SUBCLASS I. Elasmobranchii 



These have a cartilaginous skeleton, and a skin naked 

 or with placoid scales. The gill openings are uncov- 

 ered ; and the mouth is generally under the head. The 



FIG. 122. Shark (Carcharias vulgar is). Atlantic. 



ventral fins are placed far back ; the pectorals are large, 

 in the rays enormously developed ; and the tail is heter- 

 ocercal. Such are the sharks, rays, and dogfishes. 

 They are all marine. The largest shark found, and 

 therefore the largest fish, measured forty feet in length. 



SUBCLASS II. Teleostomi 



This subclass includes all of the common fishes having 

 a bony endoskeleton and a scaly exoskeleton. The 

 skull is extremely complicated ; the upper and lower 



