The Cetacean Central Nervous System

Whales, dolphins, and porpoises, 78 species of entirely aquatic mammals, comprise the order Cetacea whose distant ancestors, according to paleontologists, left land in the early Eocene about 55 to 60 million years ago. The odontoceti toothed whales, presently 67 species and the Mysticeti, II species of baleen whales, have evolved separately since at least the late Eocene. Modern cetaceans are diverse the largest blue whale is some 60,000 times heavier than the smallest newborn porpoise. Average adult size among cetaceans varies by at least 1000 times. Small and large species occupy all oceans from the equator to the polar seas, some forms also inhabit rivers, and four species live only in fresh water.