The history of the Cetacea

Whales, dolphins and porpoises are mammals that have returned to the sea, their ancestors lived on land. Therefor they are warm blooded and air breathing. Some are the largest animals that have ever lived, they can approach a length of 30.5m! Some are less than 90 cm.They are perfectly adapted to an aquatic life and are virtually incapable of leaving it.
The body of a dolphin is streamlined for efficient movement and their forelimbs are modified into flippers.
The tail has been modified to form a flat broad paddle or fluke which is used in locomotion. They also have a thick layer of blubber under the skin to protect them against the cold. The nostrils opening on the top of the head functions as a blow-hole. The position of the nostrils on top of the head is peculiar to the Cetacea. (no other mammals have similar blow-holes)

There are two living suborders of Cetacea: 

MYSTYCETI and ODONTOCETI , and one suborder to which only extinct species belong: ARCHAEOCETI (only known from fossil remains)
In the Mysticeti, the teeth, which are present as buds in the gums of the immature, never emerge. They are replaced by the baleen plates which are used to strain the food from the sea water.
Odontoceti may have a full and sometimes powerful set of permanent teeth, but unlike most mammals have no milk teeth. These beautiful and fascinating creatures are sadly becoming scarce. With the advent of industry has come to the introduction of technology into their environment, pollution and more important, "improved" methods of hunting are likely to prove their Waterloo.....
Without rigid controls on hunting, backed by international cooperation and enforcement, they may well become extinct!