144 



ZOOLOa 7. 



pointing backward, enabling tliem to seize and retain their 



prey- 

 Sharks and skates are engines of destruction, being the 



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



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



Fici. Wi.—Cestracion, or Port Jackson Shark. From LUtken'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. 



.—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 Ciestracion (Fig. 185) is an old- 

 fashioned form, which inhabits the Australian seas. 



