Order Cetacea - Whales, Dolphins, and Porpoises
The 78 living species currently recognized in the Order Cetacea are divided into 2 suborders Odontoceti (toothed whales) and Mysticeti (baleen whales). All representatives of a third suborder, Archaeoceti (ancient whales), are extinct. It is generally agreed that cetaceans are the most derived of all mammals (with the possible exception of bats). Evolved from terrestrial ancestors, they have totally adapted to living in the water, and have no need to come ashore, even for resting or reproduction.
All cetaceans share a similar general body plan: a streamlined (albeit some more so than others) spindle shaped torso; flattened paddle like foreflippers; telescoped skull; nasal openings on top (rather than on the front) of the head; a well developed blubber layer; internal reproductive organs; newly derived boneless structures in the form of tail flukes and a dorsal fin or ridge (not present in some species); and the loss of such aquatic hindrances as hind limbs (present, if at all, as vestiges), external ear flaps, and fur (although all have hair at some time during their early development and some retain a few rostral hairs for life). Although they may somewhat resemble fish externally, the cetaceans' internal anatomy betrays their terrestrial mammalian ancestry. Their flippers contain reduced counterparts of all or most of the hand and arm bones characteristic of other mammals; pelvic rudiments (and occasionally hind limb remnants) are present. The internal anatomy of cetaceans is surprisingly like that of more familiar land mammals, with such interesting exceptions as the presence of a 3 chambered stomach and cartilaginous reinforcements of the airways all the way down to the alveoli.