3836 COMPARATIVE ZOOLOGY. 
Whale feeds by putting this gigantic strainer into opera- 
tion, as it swims through the shoals of minute Mollusks, 
Crustaceans, and Fishes, which are constantly found at the 
surface of the sea. Opening its capacious mouth, and al- 
lowing the sea-water, with its multitudinous tenants, to fill 
the oral cavity, the Whale shuts the lower jaw upon the 
Fie. 333.—Troop of Dolphins, with Manatee in the distance. 
baleen plates, and, straining out the water through them, 
swallows the prey stranded upon its vast tongue.” In all 
other Cetaceans teeth are developed, especially in Dol- 
phins and Porpoises; but the Sperm Whale has them only 
in the lower jaw, and the Narwhal can show but a single 
tusk. The Dolphins are the only Mammals haying no or- 
gan of smell. 
8. Sirenians resemble the Cetaceans in shape, but are 
closely allied to the hoofed animals in organization. They 
have the limbs of the Whales, and are aquatic; but they 
are herbivorous, and frequent great rivers and estuaries. 
