144 



ZOOLOGY. 



pointing backward, enabling them to seize and retain their 

 prey. 



Sharks and skates are engines of destruction, being the 

 terror of the seas. Their entire structure is such as to 

 enable them to seize, crush, tear, and rapidly digest large 



Fig. 185.— Cestracion, or Port Jackson Shark. From Liitken's Zoology. 



fishes or other marine animals. Hence their own forms are 

 gigantic, soft, not protected by scales or armor, as they 

 have few enemies. Hence they do not need a high degree 

 of intelligence, nor special means of defence or protection, 



Fig. 186.— Mackerel Shark. 



though from their activity the circulatory system is highly 

 developed, the muscular aortic bulb being provided within 

 with three rows of semi-lunar valves. 



The eggs of sharks and rays are very large compared with 

 those of bony fishes. The Cestracion (Fig. 185) is an old- 

 fashioned form, which inhabits the Australian sea§. 



