Definition of Cetacea
: an order of completely aquatic mostly marine eutherian mammals consisting of the whales, dolphins, porpoises, and related forms, all having a very large head, a tapering body like a fish and nearly devoid of hair, forelimbs like paddles, no hind limbs, a tail ending in a broad horizontal fin, a large brain, a complex stomach of four or more chambers, and two mammae posterior in position
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Origin and Etymology of cetacea
New Latin, from neuter plural of cetaceus cetaceous
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