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.— Ces^maon, 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- 
foshioned form, which inhabits the Australiw seas. 
