The kingdom Animalia, or Metazoa, includes all animals. Animals are multicellular, eukaryotic organisms, which are heterotrophic, meaning they obtain nutrition from organic sources. Most animals obtain nutrition by ingesting other organisms or decomposing organic material.

Animal cells are characterized by their lack of a rigid cell wall exhibited by fungi and plants. Instead, animal cells are held together by structural proteins such as collagen. All animals, except for the sponges, are made up of cells organized into tissues that are specialized for some function. As a result, most animals are capable of complex behavioral responses and rapid movement.

Most animals are diploid , meaning they have two copies of all genetic information for most of their life cycle. Most animals reproduce sexually with differentiated sex cells. These cells (large, nonmotile eggs and small, motile sperm) fuse to form a new diploid individual called a zygote . The zygote undergoes a series of cell divisions, called cleavage, to form a hollow, multicellular ball known as a blastula. The blastula then folds in on itself to form a gastrula, a double-walled structure with an opening to the outside called the blastopore. Some animals (including all mammals) develop and mature directly into adults but the development of most animals includes larval stages. Larvae are immature forms that are morphologically distinct from adults. The process of metamorphosis transforms larvae into

A jellyfish is a radially symmetric animal with internal organs that are visible through its transparent, gelatinous body.
A jellyfish is a radially symmetric animal with internal organs that are visible through its transparent, gelatinous body.
their adult form. A familiar example is the metamorphosis of a tadpole into a frog.

Although taxonomists disagree about the identity of major animal groups and the relationships among them, most agree that Animalia is monophyletic. This means that all animals can trace their descent to a single common ancestor. There are approximately thirty-two living animal groups, or phyla, each with a distinctive body plan and biological properties.

All of these are the survivors of the one hundred or so animal phyla that evolved during the Cambrian explosion. This incredible diversity of animal body plans and lifestyles arose in the relatively short period of 40 million years, between 565 and 525 million years ago. All of today's remarkably diverse animal forms are variations on the basic body plans that evolved during the Cambrian.

The most primitive animal group is the phylum Porifera, the sponges. The remaining animal groups can be divided into radial and bilaterally symmetric animals. Radially symmetric animals are the cnidarians, including jellyfish, corals, and anemones, and ctenophores, or comb jellies. Bilaterally symmetric animals (which include all vertebrates) are further divided based on types of body cavities and variations on the pattern of gastrula formation during development. Flatworms, phylum Platyhelminthes, have no body cavity. Ten phyla of animals, including nematodes and rotifers, have a primitive type of body cavity.

All other animals have a true body cavity and are divided into two major groups. Protostomes include Mollusca (clams, snails, and octopi), Annelida (segmented worms), Arthropoda (spiders, crustaceans, and insects), and several minor phyla. Deuterostomes include Echinodermata (sea stars and sea urchins), two proto-chordate phyla, and Chordata (tunicates, lancelets, sharks, fish, amphibians, snakes and lizards, birds, and mammals).

SEE ALSO Annelid ; Arthropod ; Cambrian Explosion ; Cell ; Chordata ; Cnidarian ; Echinoderm ; Mollusk ; Nematode ; Platyhelminthes ; Porifera ; Protein Structure ; Tunicate

Tanya A. Dewey


Campbell, Neil A., Jane B. Reece, and Lawrence G. Mitchell. Biology. Menlo Park, CA: Benjamin/Cummings, 1999.

Hickman, Cleveland P. Jr., Larry S. Roberts, and Allan Larson. Animal Diversity. New York: McGraw-Hill, 2000.

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