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sirenian[sIrE´nEun] Pronunciation Key or sea cow, name for a large aquatic mammal of the order Sirenia. Living sirenians are the dugong and the manatee, both found in warm, shallow waters in sheltered regions, where they feed on seaweeds and sea grasses. Sirenians are the only marine mammals, outside of the whale order, that spend their entire lives in the water, and they are the only marine mammals that feed exclusively on vegetation. Their heavy, thickset bodies are fishlike in form, the tail ending in a horizontally flattened fin. There are no hind legs, and the forelegs are modified into weak flippers. The gray skin is completely hairless, except for the bristles around the fleshy lips; the upper lip is cleft into two lobes, used for gathering food. The female has a pair of mammary glands on the chest, and holds the pup in her flippers while nursing. It has been speculated that the manatees, which nurse on the water's surface, are the source of mermaid legends.
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