Coelenterata
| Coelenterata | |
|---|---|
| |
| Comb jellies (Beroe spp.) | |
| Scientific classification | |
| Kingdom: | |
| Subkingdom: | |
| (unranked): | Coelenterata
|
| Phyla | |
Coelenterata is an obsolete yet common term encompassing two animal phyla, the Ctenophora (comb jellies) and the Cnidaria (coral animals, true jellies, sea anemones, sea pens, and their allies). The taxon name comes from the Greek "koilos" ("hollow"), referring to the hollow body cavity common to these two phyla. They have very simple tissue organisation, with only two layers of cells, external and internal.
Corals, jellyfish, hydras, and sea anemones are all coelenterates. In addition to having stinging cells, these invertebrates have other characteristics that make them unique. Coelenterates have no bones. Instead, they have a double layer of cells making up their body. The outside layer is the outside of their body, and the inside is the lining of the body cavity, where they digest their food. Some coelenterates, like the jelly fish, have a third layer of cells in between the other two. This layer helps give shape to their body. They have one opening in their body for both food and wastes.
The stinging cells of coelenterates are located in tentacles, long "arms" attached to the rim of their body opening. When a small animal touches these tentacles, the stinging cells send sharp barbs into the animal, releasing their poison. Then the tentacles move the animal toward the body opening. Some coelenterates have only five or six tentacles. Others have hundreds.
A coelenterate's body has radial symmetry. An object with radial symmetry has all its parts arranged in a circle around a center point. The picture shows the radial symmetry of a hydra.
Scientists classify coelenterates into three main classes. Jelly fish belong to the class Scyphozoa; hydras belong to the class Hydrozoa; and corals and sea anemones belong to the class Anthozoa.
History of classification
The term coelenterate is no longer recognized as scientifically valid, as the Cnidaria and Ctenophora have placed at equal rank under the Metazoa with the other phyla of animals.[1] A single term encompassing these two phyla but leaving out all others of equal rank would be considered polyphyletic. 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[2]. (The latter refers to all the animals except the sponges, Trichoplax, and the still poorly-understood Mesozoa.) Neither grouping is accepted univerally[3]; however, both are commonly encountered in taxonomic literature.
References
- ^ 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 0613923383
- ^ NCBI Taxonomy Browser