The order Cetacea is composed of the whales, dolphins and porpoises and is divided into two suborders: Odonotcetes (whose representatives have asymmetrical skulls and teeth that are ideal for seizing fleeing prey) and Mystecetes (whose representatives have symmetrical skulls and baleen to strain planktonic organisms from the water). There is some dispute regarding the species placement within some of the families and genera of this order. This account is taken from Walker’s Mammals of the World, Vol.2 6th Edition, which identifies 13 Recent families, 41 genera and 78 species. The cetaceans are distributed in marine environments worldwide as well as some lakes and rivers that are completely fresh water. The geological range of Cetacea is from early Eocene to Recent.

Cetaceans are completely aquatic, a fact that separates them from all other mammals. They are well adapted to life in the water. Their smooth streamlined bodies are virtually, if not completely hairless. They conserve body heat with a thick insulating layer of blubber beneath their skin. Their front limbs are flippers and their hind limbs are absent. They propel themselves through the water using an up and down motion with their flattened, horizontal tail. (Certain cetaceans are among of the fastest swimmers in the oceans, with some dolphin species able to maintain speeds of 26-33 km/hr.) The nostrils have moved to the top of the head, and in Odontocetes have become a single blowhole; whereas in Mystecetes it is double. The circulatory and respiratory systems of cetaceans are physiologically adapted to allow them to perform prolonged dives, sometimes at great depths. Hearing is highly developed in cetaceans. Odontocetes use echolocation to navigate and find food. This ability does not seem to be present or at least as developed in Mystecetes.