Criteria Used for Animal Classification

Animals show enormous diversity, and biologists use several structural yardsticks to bring order to this diversity and place species into a natural classification scheme.

Grades of organisation. The simplest animals show a cellular level of organisation, where the body is essentially a loose aggregation of cells with little cooperation between them. A step up is the cell-tissue level, where some cells combine to perform a shared function. Most animals reach the tissue-organ level, where tissues combine into organs, and the most complex forms show an organ-system level, where organs work together in coordinated systems.

Body symmetry. An animal may be asymmetrical (no definite plane divides it into equal halves), radially symmetrical (any plane passing through the central axis divides the body into similar halves — well suited to animals that are stationary or float), or bilaterally symmetrical (only one plane, through the midline, divides the body into mirror-image right and left halves — usually associated with a definite head end and active, directional movement).

Body cavity. Triploblastic animals (see below) may lack a true fluid-filled body cavity between the gut and body wall (acoelomate), possess a body cavity that is not fully lined by mesoderm (pseudocoelomate), or possess a true body cavity completely lined by mesoderm on both sides (coelomate, or eucoelomate).

Germ layers. Animals whose embryo develops from two primary germ layers — an outer ectoderm and inner endoderm — are diploblastic. Animals that develop a third, middle germ layer, the mesoderm, are triploblastic; the mesoderm gives rise to most organs and is essential to the higher grades of organisation.

Segmentation. In some animals the body is a single continuous unit (unsegmented), while in others it is divided into a linear series of similar segments, each repeating some body structures (segmented, or metameric).