Animals

All organisms in the animal kingdom are multicellular heterotrophs, and their cells lack cell walls. Most animals reproduce sexually, have some type of nervous system and are capable of movement.

There are over 1.5 million known species of animals. This branching tree shows how major animal groups are possibly related.

Invertebrates are animals without backbones. These animals live in water and on land. They account for over 95% of the known animal species.

Some of the major groups of invertebrates include sponges, worms, mollusks, echinoderms, cnidarians and arthropods.

Vertebrates have a spinal cord encased by protective vertebrae. They also have an endoskeleton that is composed of cartilage or bone. Fish, amphibians, reptiles, birds and mammals are all vertebrates.