Animals show various patterns in their morphology. Relationships between animals or groups of animals are best explained by their cooperative morphology and embryology. The gross external morphology of animals falls under a limited number of patterns (criteria). ? These include form of animals (symmetry), ? arrangement of body parts in segments (metamerism), ? formation of a head (cephalization), and progressive sequence of specialization of structure (levels or grades of organization). Similary, the criteria of internal morphology are differences in formation of body cavity (coelom) and reproduction (embryology), etc. Symmetry and its significance: Symmetry means an arrangement of body parts into geometrical designs. It refers to the division of body into equal parts by lines or planes. 1. An animal is called symmetrical when a plane passing through its centre will divide into similar halves. When an animal cannot be divided into like parts by a plane, it is called asymmetrical e.g. sponge, some Protozoa (Amoeba:) and few others. 2. Certain terms are often used when explaining symmetry. 3. An axis is an imaginary line passing through the center of body, such as longitudinal axis and oral - aboral axis. Either end of an axis is termed a pole. Thus, each axis has two poles. 4. A plane of symmetry is a straight line that divides into corresponding halves. 5. Metazoa commonly display two types of symmetry, radial and bilateral. Two other types of symmetry are also recognized, spherical and biradial. 6. Protozoa are not only asymmetrical but display all four types of symmetry in their diverse body forms.