Coelenterata is a term encompassing the animal

phyla Phyla, the plural of ''phylum'', may refer to: * Phylum, a biological taxon between Kingdom and Class * by analogy, in linguistics, a large division of possibly related languages, or a major language family which is not subordinate to another Phyl ...

Cnidaria Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in Fresh water, freshwater and Marine habitats, marine environments, predominantly the latter. Their distinguishing feature is cnidocyt ...

(

coral Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and secre ...

animals, 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), and radial symmetry. Some examples are

s, which are typically colonial, and hydrae,

jellyfish Jellyfish and sea jellies are the informal common names given to the medusa-phase of certain gelatinous members of the subphylum Medusozoa, a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animals with umbrella- ...

, and sea anemones, which are solitary. 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

tentacle In zoology, a tentacle is a flexible, mobile, and elongated organ present in some species of animals, most of them invertebrates. In animal anatomy, tentacles usually occur in one or more pairs. Anatomically, the tentacles of animals work main ...

s equipped with either nematocysts or colloblasts to capture mostly

plankton Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a cr ...

ic 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 network of nerves is spread throughout the body. Many Cnidaria exhibit

polymorphism Polymorphism, polymorphic, polymorph, polymorphous, or polymorphy may refer to: Computing * Polymorphism (computer science), the ability in programming to present the same programming interface for differing underlying forms * Ad hoc polymorphis ...

, 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 rejected, as the Cnidaria and Ctenophora have less in common than previously assumed. Coelentera may only be monophyletic if both Placozoa and

Bilateria The Bilateria or bilaterians are animals with bilateral symmetry as an embryo, i.e. having a left and a right side that are mirror images of each other. This also means they have a head and a tail (anterior-posterior axis) as well as a belly an ...

are included. In particular, the phylogenetic position of Ctenophora is controversial; it was first considered a sub-group of coelenterata but Hyman regarded it as a separate phylum. Most researchers think that Coelenterata is not

monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic ...

, and therefore any group containing Cnidaria and Ctenophora but excluding other phyla would be

paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...

. Previously, 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.

References

{{Authority control Obsolete animal taxa Cnidarian taxonomy