Mammals have backbones, are warm-blooded, breathe air and whose females have milk-secreting glands for their young. There are over 5,000 species of mammals, ranging from the 2-inch shrew to the 120-foot whale.
An invertebrate is an animal without a backbone. Invertebrate animals include fruit flies and sea sponges.
Reptiles are vertebrates that regulate body temperature externally, have dry, glandless skin covered with scales, breathe through lungs and lay large eggs that develop on land.
Amphibians are cold-blooded, smooth-skinned vertebrates. They are unable to regulate their own body heat, depending on sunlight to become warm and active. Some live on land and some in the water, most returning to the water to mate and lay eggs.
Fish are cold-blooded aquatic vertebrates usually having scales and breathing through gills. There are over 27,000 species of fish, making them the most diverse group of vertebrates.
Birds are warm-blooded, egg-laying, feathered vertebrates with a backbone and skeleton with some of their bones hollow to keep them light and aloft. Their forelimbs have the same bones as the human arm (highly modified to form the structure for wings).