1 84 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



ing its capacious mouth, and allowing the sea water, 

 with its multitudinous tenants, to fill the oral cavity, the 

 whale shuts the lower jaw upon the baleen plates, and 

 straining out the water through them, swallows the prey 

 stranded upon its vast tongue." In the other cetaceans 

 teeth are developed, especially in dolphins and porpoises ; 

 but the sperm whale has them only in the lower jaw, 

 and the narwhal can show but a single tusk. In the 

 toothed cetaceans the organ of smell is very rudimentary 

 or even absent (dolphins). 



3. Sirenia resemble the cetaceans in shape, but are 

 closely allied to the hoofed animals in organization. 



FIG. 172. Troop of dolphins, with manatee in the distance. 



They have the limbs of the whales, and are aquatic ; but 

 they are herbivorous, and frequent great rivers and 

 estuaries. They have two sets of teeth, the cetaceans 

 having but one. They have a narrow brain ; bristles 

 scantily covering the body ; and nostrils placed on the 



