They have feathers and a bill. Teeth have been lost in the birds. They share several derived features with crocodiles that unite both groups in a cladistic classifiation. The mammals have hair or fur. They also possess mammary glands.
Most species have live birth, but a few marsupials lay eggs.
The overall shape and form of the animal will reflect its adaptation to the environmental conditions in which it lives. However, this divergence in habitat use and feeding behavior usually occurs through the modification of existing structures rather than by developing completely new structures. Species that differ in their use of habitat or food resources will tend to possess modified homologous structures. You should examine the displays showing vertebrate divergence in skeletal structure and limb morphology, mammal divergence in tooth morphology, and bird divergence in beak and foot morphology. The adults are ectoparasites of other fish.