Cnidarians are radially or biradially symmetric, a general type of symmetry believed primitive for eumetazoans. They have achieved the tissue level of organization, in which some similar cells are associated into groups or aggregations called tissues, but true organs do not occur. Cnidarian bodies have two or sometimes three layers. Often tentacles surround the opening. Some cells are organized into two simple nerve nets, one epidermal and the other gastrodermal, that help coordinate muscular and sensory functions.
Cnidarians have two basic body forms, medusa and polyp.
Medusae , such as adult jellyfish, are free-swimming or floating. The mouth is usually on the concave side, and the tentacles originate on the rim of the umbrella. Polyorchis penicellatus , from the Order Hydroida, with medusa form. Polyps , in contrast, are usually sessile. Syncoryne micrabilis , with polyp body form.