They are all nourished with their mothers' milk and require maternal care and attention in the early stages of their life. Birds are feather-bearing and lay eggs. Fishes are scale-bearing and also lay eggs. They all live in either salt water or fresh water. They don't breath air with lungs but rather extract the oxygen in the water through gills.
Reptiles are scale-bearing and again, many lay eggs.
Amphibians have smooth and moist skin and they lay eggs without shells. Other animals in the categories below do not have a spinal column. Crustaceans, or shellfish, live in water, with few exceptions, such as the wood louse. Their skeleton is their skin, and this is why it is called an exoskeleton. The exoskeleton is not made of bone but rather of chitin.