146 



FIRST LESSONS IN ZOOLOGY. 



in the larger, upper lobe. They also have from five to 

 seven gill-openings or slits, whereas the cod or perch has 

 but one. The skin is either smooth, or with minute scales, 



forming shagreen. Both jaws are 

 armed with numerous sharp, flat- 

 tened teeth, arranged in rows and 

 pointing backward, enabling them 

 to seize and retain their prey. 



With the appearance of sharks 

 the world of life realized a new 

 order of things. Never before had 

 animals lived so well adapted for 

 the destruction of the lower orders 

 of animals, however well protected 



^ ie y were ky solid sh 6 ^ 8 an( l other 

 means of protection. Sharks and 

 FIG. i58.-Teeih of the Ces- skates are engines of destruction, 

 tracion. being the terror of the seas. Their 



entire structure is such as to enable them to seize, crush, 

 tear, swallow, and rapidly digest large fishes, shell-fish, star- 

 fish, sea-urchins, or other marine animals. Moreover their 

 own forms are gigantic, soft, not protected by scales or 



FIG. 154. Carcharias. 



armor, as they have few enemies. Hence they do not need 

 a high degree of intelligence, nor special means of defence 

 or protection, though from their activity, having large, 

 powerful fins, the circulatory system is highly developed, 

 the heart being more complicated than in bony fishes. The 



