Cnidaria. found exclusively in aquatic and mostly marine environments
Coelenterata is term encompassing animal phyla, the Cnidaria (coral animals, true jellies, sea anemones, sea pens, and their allies). Ctenophora was first considered a sub-group but Hyman regarded
Ctenophora as a separate phylum. 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 examples are corals, which are colonial, and hydra, jelly fish and sea anemone which are solitary.
Coelenterata lack a specialized circulatory system, relying instead on diffusion across the tissue layers.
All coelenterates are aquatic, mostly marine. The bodyform is radially symmetrical. The body has a single opening, the hypostome, surrounded by sensory tentacles equipped with either nematocysts
or colloblasts 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. Coelentera may only be
monophyletic if both Placozoa and Bilateria are included. 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
Aufbau aus lediglich zwei Zellschichten Epidermis und Gastrodermis, weshalb man auch von diploblastischen Tieren spricht. Zwischen diesen beiden Zellschichten liegt bei beiden Tiergruppen eine
gallertartige Zone, das so genannte Mesenchym.