Coelenterata

Coelenterata
Comb jellies (Beroe spp.)
Scientific classification
Kingdom:	Animalia
Subkingdom:	Eumetazoa
(unranked):	Coelenterata
Phyla
Ctenophora Cnidaria

Coelenterata is term encompassing two animal phyla, the Ctenophora (comb jellies) and the Cnidaria (coral animals, true jellies, sea anemones, sea pens, and their allies). The name comes from the Greek "koilos" ("hollow bellied"), referring to the hollow body cavity common to these two phyla. They have very simple tissue organization, with only two layers of cells, external and internal and radial symmetry. Some of the examples are corals, sea anemone which are colonial and hydra, jelly fish which are solitary.

Characteristics

All coelenterates are aquatic, mostly marine. The bodyform is radially symmetrical. The body has a single opening hypostome surrounded by sensory tentacles equipped with either nematocysts or collobasts to capture mostly planktonic prey. These tentacles surround a spacious cavity called the gastrovascular cavity or coelenteron. Digestion is both intracellular and extracellular. Respiration and excretion are accomplished by simple diffusion. A network of nerves is spread throughout the body. Many forms 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.

History of classification

The scientific validity of the term coelenterate is currently disputed, as the Cnidaria and Ctenophora have less in common than previously assumed. In particular, the phylogenetic position of Ctenophora is controversial, leading some researchers to suggest that Coelenterata is not monophyletic, and therefore any group containing Cnidaria and Ctenophora but excluding other phyla would be paraphyletic. However, some genomic studies have found support for monophyletic coelenterates. Despite this uncertainty, the term coelenterate is still used in informal settings to refer to the Cnidaria and Ctenophora.

Complicating the issue is the 1997 work of Lynn Margulis (revising an earlier model by Thomas Cavalier-Smith) that placed the Cnidaria and Ctenophora alone in the branch Radiata within Eumetazoa. (The latter refers to all the animals except the sponges, Trichoplax, and the still poorly understood Mesozoa.) Neither grouping is accepted universally; however, both are commonly encountered in taxonomic literature.