INTRODUCTION. 



The Sirenia, owing to their aquatic life and fish-like shape, were 

 regarded by the Cuviers and their immediate successors as belonging 

 to the order Cetacea, and were named Herbivorous Cetacea. Further 

 observations into their habits and structure have shown that they 

 differ materially from the Cetacea, and they have had to be removed 

 therefore from that order. As they also differ much from other 

 orders of Mammals, and cannot be classified with any one of them, 

 these animals have to be placed in an Order apart, and the name 

 Sirenia, proposed by Illiger, has been adopted by zoologists. 



As regards their habits, they live in the brackish water of large 

 rivers, estuaries, bays, lagoons, and do not frequent the open sea. 

 They feed on the seaweeds and other aquatic plants which groAV in 

 these waters. They are deliberate and inactive in their movements. 

 They frequent the east and west coasts of Africa, the Red Sea, the 

 Bay of Bengal, the Malay Archipelago, the north coast of Australia, 

 the West Indies, some of the great rivers of South America, and up 

 to the latter half of the eighteenth century one species lived in the 

 Behring Sea. 



In their external characters, the skin is not smooth and shining 

 as in the Cetacea, but is rough, wrinkled and sparingly studded with 

 hairs ; the muzzle is truncated, and on its summit the nose opens 

 by two orifices ; the lips are provided with short, stiff bristles ; the 

 body is fusiform ; the palpebral fissure and eye are very small ; the 

 ear has no pinna ; there is no dorsal fin ; the caudal fin is horizontal, 

 laterally expanded, and in Halicore has a notch on the posterior 

 border ; the pectoral limbs are pentadactylous, enclosed in a common 

 envelope of skin, paddle-shaped ; the pelvic bones are present, but 

 rudimentary ; there are no hind limbs. The head is not large in 

 relation to the size of the body. 



In their internal structure, the brain is smaller than in the active- 

 moving Cetacea of corresponding size, and the cerebrum is not so 

 minutely convoluted. The heart is cleft at the apex. The epiglottis 

 is not intranarial. The stomach consists of two large compartments, 

 oesophageal and pyloric, and of two intermediate accessory cavities ; 

 a special glandular pouch opens into the oesophageal compartment. 

 An ileo-colic valve marks the separation of the small from the large 



14.3 



