Coelenterata

Coelenterata is a rejected phylum encompassing the animal phyla Cnidaria (corals, true jellies, sea anemones, sea pens, and their relatives) and Ctenophora (comb jellies). The name comes , referring to the hollow body cavity common to these two phyla. They have very simple tissue organization, with only two layers of cells (ectoderm and endoderm), along with a middle undifferentiated layer called the mesoglea, and radial symmetry. Coelenterata lack a specialized circulatory system, relying instead on diffusion across the tissue layers.

==Characteristics== All coelenterates are aquatic, mostly marine, animals. The body form is radially symmetrical, diploblastic and does not have a coelom. The body has a single opening, the '', surrounded by sensory tentacles equipped with either nematocysts or colloblasts to capture mostly planktonic prey. These tentacles are surrounded by a spacious cavity called the gastrovascular cavity, or coelenteron. Digestion is both intracellular and extracellular. Respiration and excretion are accomplished by simple diffusion. A nerve net is spread throughout the body. Many Cnidaria exhibit polymorphism, wherein different types of individuals are present in a colony for different functions. These individuals are called zooids. These animals generally reproduce asexually by budding, though sexual reproduction does occur in some groups.