The fundamental structural unit of all living things is the cell, consisting of a fluid material called cytoplasm, surrounded by a membrane. The cells of an animal are organized into groups called tissues. All the cells of any one kind of tissue are similar in structure, and are specialized for one or more particular functions. Several kinds of tissues may form an organ, such as the eye, heart, or stomach.

The various organs of animals are usually associated in organ systems. Most invertebrates (animals without backbones) and all vertebrates (animals with backbones) have the following organ systems:

This system consists of the skin and other body covering. The covering protects inner tissues from injury and disease. It usually consists of several layers. Feathers, scales, and hair are part of the body coverings of various animals.

The skeleton supports the body, and in many animals works with the muscular system during movement. Some animals—such as insects, lobsters, shrimp, clams, and snails—have exoskeletons (external skeletons). Exoskeletons are composed of layers of proteins and of a non-protein material, called chitin, containing some calcium salts. In clams, snails, and most other mollusks, the exoskeleton is a hard shell having a higher proportion of calcium salts. Vertebrates have endoskeletons (internal skeletons) composed of cartilage or cartilage and bone.

The movements of an animal's body, as a whole or in parts, are achieved by the contraction and relaxation of its muscles.

Food provides animals with materials necessary for energy, growth, and manufacture of new cells. It is broken down by a process called digestion into chemical substances that the body can use. These substances are then assimilated, or absorbed, into the body cells. The digestive system of complex animals includes the mouth, esophagus, stomach, intestines, and glands.

Oxygen is taken into animal cells, and carbon dioxide is expelled. The exchange of the two gases is called respiration. Most species of animals have a system of respiratory organs, which include air holes such as nostrils or spiracles, tubes for conducting air, and organs—such as gills or lungs—in which the actual exchange of gases takes place.

Food, oxygen, and certain gland secretions are moved through the body to all cells, and waste materials are carried away from the cells. In all animals except the more simple ones, food is carried to the cells and waste carried away through the circulatory system, which includes the heart and blood vessels.

Through the immune system, most animals have the capacity to resist the development of diseases. In vertebrates, the immune system is also called the lymphatic system; it includes the lymph nodes, lymphatic vessels, spleen, and thymus gland.

Animals excrete (eliminate) waste materials. Carbon dioxide is excreted through the respiratory system. Excess water and other materials may be excreted directly through the body covering in the simpler animals. Complex animals have a system of excretory organs, including the kidneys, the ureter, and the bladder.

The glands of internal secretion, or endocrine glands, help regulate body functions. They secrete substances called hormones, some of which have stimulating, others inhibiting, effects on body processes.

Most animals have a system of especially responsive organs—including the brain, nerves, and sense organs—that (together with the endocrine system) regulate body functions.

All living things are capable of reproducing their own kind. Some invertebrates reproduce asexually by various methods, such as division of the parent cell into separate individuals. All vertebrates and many invertebrates reproduce by the joining of specialized cells called gametes, one from a male and one from a female parent. This process is called sexual reproduction.

The bodies of most kinds of animals are symmetrical in the arrangement of their parts. Radially symmetrical animals are those whose body parts radiate from a central point. Echinoderms (such as starfish and sea urchins) and cnidarians (such as hydra and corals) are radially symmetrical. Most animals, including humans, are bilaterally symmetrical; that is, a plane passing from top to bottom and back to front through the center would divide the body into two halves that, except for certain internal organs, are approximate mirror images of each other.