A polyp is described as a cylindrical organism with a thin body-wall consisting of two single layers of cells, the ectoderm and the endoderm separated by a gelatinous noncellular layer or mesogloea.

At one end there may be a mouth which is usually surrounded by tentacles. Polyps may be single (e.g, Hydra) or colonial (e.g. the coral-forming organisms). The latter are formed by repeatd budding from a parent polyp.

In such colonies a tube or coenosarc links the body-cavities of all the individuals with one another. There may be many different kinds of polyp in a colony, each specialized for some different function; such as, feeding, reproduction, protecting, etc.