|Comb jellies (Beroe spp.)|
Coelenterata is an obsolete 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" ("full 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.
All coelenterates are aquatic, mostly marine. The bodyform is radially symmetrical with a tissue grade of organisation. The body has a single opening hypostome surrounded by sensory tentacles leading into 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 term coelenterate is no longer recognized as scientifically valid, as the Cnidaria and Ctenophora are placed at equal rank under the Metazoa with the other phyla of animals. A single term encompassing these two phyla but leaving out all others of equal rank would be considered paraphyletic. Nonetheless, 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 under the Radiata branch of the Eumetazoa subregnum. (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.
- Excerpt from Britannica article regarding Ctenophore classification
- Margulis, Lynn and Karlene V. Schwartz, 1997, Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth, W.H. Freeman & Company, ISBN 0-613-92338-3
- NCBI Taxonomy Browser