Telling Animals From Other Organisms

Most biologists classify organisms into five kingdoms: monera, protists, fungi, plants, and animals. It is not always obvious to which kingdom an organism belongs. For example, some protists (such as slime molds) can move from place to place under their own power—a trait normally associated with animals. However, not all such protists are multicelled (composed of more than one cell), whereas all animals are. Another example is the sponge, which is immobile as an adult and may seem to be a plant. However, biologists place the sponge in the animal kingdom because it is mobile while immature and has a cell structure like an animal. In deciding whether an organism is an animal, biologists consider a number of characteristics, including the following:

  • All animals are multicelled, distinguishing them from monera and most protists.
  • The cells of animals lack a cell wall, a rigid structure that encloses plant and fungus cells. Animal cells are enclosed in a pliable structure, the cell membrane.
  • Most animals can move about under their own power throughout life, and all animals are mobile during at least one phase of their life. Plants and fungi are incapable of moving from one spot to another under their own power.
  • Animals obtain food by eating other organisms. Plants manufacture their food from inorganic substances by a process called photosynthesis. Fungi absorb nutrients from decaying organic matter.
  • Animals have a greater degree of irritability, or responsiveness, than do other organisms. Most kinds of animals have nervous systems, which enable them to respond rapidly to stimulation.
A classification of the animal kingdom
PhylumCharacteristics
Porifera (Sponges) Sponges attach themselves to rocks and other objects at the bottom of oceans, lakes, or rivers. Many take the shape of such objects. Sponges have cells called choanocytes or collar cells that trap food particles within chambers in their bodies.
Cnidaria (Cnidarians or coelenterates) Cnidarians may be shaped like a cylinder, a bell, or an umbrella. Their bodies contain a jellylike material between two layers of cells. This phylum includes jellyfish, sea fans, sea anemones, and corals.
Ctenophora (Comb jellies or sea walnuts) These transparent animals live in oceans. They have eight bands of comblike organs on the side of their bodies. Most are pea-sized to thimble-sized. Comb jellies of a group called Venus' girdle can measure over 3 feet (90 centimeters) long.
Platyhelminthes (Flatworms) Many flatworms live as parasites in other animals. Flatworms have soft, thin, flattened bodies with three layers of cells. Most are less than 1 inch (2.5 centimeters) long. The largest flatworms, called tapeworms, grow as long as 100 feet (30 meters). Some flatworms cause such diseases as schistosomiasis.
Nemertea (Ribbon worms or proboscis worms) Almost all ribbon worms live in the oceans. They have a slender, often flattened, body. The worms shoot out a proboscis (tubelike structure) from their head to capture prey. Some species can inject poison from the proboscis into their prey. Most of these worms range from less than 1 inch (2.5 centimeters) to 8 inches (20 centimeters) long, but one species can reach a length of 100 feet (30 meters).
Rotifera (Rotifers or "wheel animals") Rotifers live in lakes, rivers, streams, and the oceans. They have cylinder- or vase-shaped bodies. On their heads are circles of hairlike projections known as cilia. The largest rotifers are about 1/10 inch (3 millimeters) long.
Acanthocephala (Spiny-headed worms) These parasites live in many animals. They have a spiny, tubelike proboscis on their head that attaches them to the wall of their hosts' intestines. Most species measure about 3/4 inch (2 centimeters) or less in length.
Nematoda (Roundworms or nematodes) Many roundworms live in soil, water, or dead tissue. Some are parasites that are found in living plants and animals. Roundworms range from microscopic to about 3 feet (90 centimeters) long. The phylum includes filariae, hookworms, pinworms, and trichinae. Parasitic species cause such human diseases as trichinosis, elephantiasis, and filariasis.
Mollusca (Mollusks) Mollusks make up the largest group of water animals, though some species live on land. Most mollusks have a hard shell that protects a soft body. The phylum includes clams, mussels, octopuses, oysters, snails, and squids.
Annelida (Segmented worms) The bodies of these worms consist of segments. Many of these worms have tentacles on their head and a pair of leglike projections called parapodia on each body segment. Earthworms and leeches belong to this phylum.
Arthropoda (Arthropods) Arthropods have jointed legs, segmented bodies, and an outside shell called an exoskeleton. This phylum includes insects, such as ants, bees, beetles, and butterflies; crustaceans, such as crabs, lobsters, and shrimps; arachnids, such as mites, ticks, and spiders; centipedes; and millipedes.
Bryozoa (Bryozoans) Bryozoans live in water, and most form colonies. Some colonies are jellylike masses. Others form branchlike networks on water plants. Bryozoans have a boxlike or tube-shaped body that holds fluid. Tentacles (feelers) cluster on the head.
Brachiopoda (Lamp shells) Lamp shells have two hard shells that cover a soft body. They live in the oceans. Some attach themselves to rocks and other hard surfaces. Others burrow or lie loose in sand or mud.
Chaetognatha (Arrow worms) These worms have an arrow shape. They range from about 1/4 to 6 inches (0.5 to 15 centimeters) long. They have movable hooks on their head that they use to catch prey. They live in open seas, particularly in warm waters.
Echinodermata (Echinoderms) Echinoderms are spiny-skinned animals that have an internal skeleton made of calcium. They are the only animals that possess tiny tubelike structures called tube feet. This phylum includes brittle stars, sand dollars, sea cucumbers, sea urchins, and starfish.
Chordata (Chordates) At some point in their life cycle, all chordates have a notochord (a rodlike, flexible cord that runs down the back of the body). A hollow nerve tube runs above the notochord. This phylum is the one to which human beings and many familiar animals belong. It includes amphibians, birds, mammals, and reptiles, as well as hagfishes, lampreys, and bony fishes.