Animals are multicellular heterotrophic organisms that lack cell walls. Some animals, called vertebrates, have a backbone. Other animals, called invertebrates, do not have a backbone. Invertebrates account for more than 95 percent of all animal species alive today. Most members of the animal kingdom share other important characteristics, including sexual reproduction and movement.

Multicellular Organization

The bodies of animals are multicellular. Some animals contain large numbers of cells. For example, some scientists estimate that the body of an adult human contains 50 trillion to 100 trillion cells. Unlike the cells of unicellular organisms, the cells of multicellular organisms do not lead independent lives. Each cell depends on the presence and functioning of other cells.

In all but the simplest animal phyla, there is a division of labor among cells. Specialization is the evolutionary adaptation of a cell for a particular function. Just as a general contractor makes use of carpenters, electricians, and plumbers to build a house, a multi-cellular organism makes use of specialized cells to perform particular functions, such as digesting food, removing wastes, or reproducing.

A tissue is a group of similar cells that perform a common function. Most animal bodies are composed of combinations of different kinds of tissues. The formation of tissue from many individual cells is made possible by cell junctions, connections between cells that hold the cells together as a unit. The members of most animal phyla have organs, body structures that are composed of more than one type of tissue and that are specialized for a certain function.

Without multicellularity, the enormous variety found in the animal kingdom would not exist. The size of unicellular organisms is limited. Moreover, all of their functions, such as reproduction and digestion, must be handled within a single cell. Multicellularity and cell specialization have enabled organisms to evolve and adapt to many environments.

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