That group of the Radiata whose members typically bear tentacles and possess intrinsic nematocysts. The name Cnidaria is also used for this phylum and is preferred by some because the name Coelenterata, as first used, included the sponges (Porifera) and the comb jellies (Ctenophora), as well as the animals called coelenterates. See also Ctenophora; Porifera.
The coelenterates are mainly marine organisms and are best known as jellyfish or medusae, sea anemones, corals, the Portuguese man-of-war, small polypoid forms called hydroids, and the fresh-water hydras. Taken together, the phylum is divisible into three classes as follows: (1) Hydrozoa, the hydroids, hydras, and hydrozoan or craspedote jellyfish (hydromedusae); (2) Scyphozoa, the acraspedote jellyfish; and (3) Anthozoa, the sea anemones, corals, sea fans, sea pens, and sea pansies. See also Anthozoa; Hydrozoa; Scyphozoa.
It is convenient to recognize two basic body forms in this phylum, the polyp and the medusa, into which all coelenterates can be classified. The polyp and the medusa, however, have many features in common (see illustration).
Comparison of hydroid polyp, medusa (inverted), and anthozoan polyp. (After T. I. Storer and R. L. Usinger, General Zoology, 3d ed., McGraw-Hill, 1957)
The polyp is a radially, biradially, or radiobilaterally symmetrical individual having a longitudinal oral-aboral axis and is usually sessile. The mouth is at the free end and is surrounded by one to many whorls or sets of tentacles which may be hollow or solid. The aboral end is commonly developed as an adhesive device for attachment and is conveniently referred to as a base. The central body cavity is the gastrovascular cavity, also called the enteron or coelenteron.
The medusa is a tetramerously or polymerously radial individual and is free-swimming. The body is usually bell- or bowl-shaped with the mouth suspended in the center of the underside of the bell on a stalk. Instead of directly surrounding the mouth as in the polyp, the tentacles are located at the margin of the bell. The outer or aboral part of the bell is recognized as the exumbrella and the under or oral part as the subumbrella. The mouth leads to the central stomach which in turn gives rise to four or more radial canals. These radial canals run through the umbrella, on the subumbrellar side, and commonly lead to a ring canal at the margin which is continuous around the margin.
The unique and most distinctive feature of coelenterates is the possession of intracellular, independent effector organelles called nematocysts, but also known as stinging cells or nettle cells. A coiled thread tube in each cell may be rapidly everted under proper stimulation and used for food gathering and for defense against predators, intruders, or enemies. Nematocysts are produced within cells called cnidoblasts. The morphologically simplest coelenterates, the Hydrozoa, have nematocysts limited to their outer epidermis whereas the more complex Scyphozoa and Anthozoa bear nematocysts in both the outer epidermis and inner gastrodermis.
The body systems of coelenterates may be divided into the following categories for the purpose of description: epithelial, nervous, muscular, mesogleal, skeletal, digestive, and reproductive. There is no analog of a circulatory system and the functions of respiration and excretion are carried out by each cell.
The epidermis is usually ciliated in polyps so that the net effect of the ciliary beat is to move material away from the mouth toward the margin of the oral disk and toward the tips of the tentacles. The gastrodermis, or inner epithelial layer, is also abundantly ciliated. These cilia move food materials about within the gastric cavity and circulate the contained water. One of the prime functions of the gastrodermis is digestion, and essentially every cell of the gastrodermis is capable of ingesting small food particles.
The coelenterate nervous system can be defined as consisting basically of an unpolarized network of bipolar and multipolar neurons. The nervous system is subepithelial in location, both subepidermal and subgastrodermal, and may in some forms consist of two networks in only limited contact with one another and with each specialized either for rapid through conduction or for slower, more general spread of conduction.
The muscular system varies considerably among coelenterates. In simple polyps there is an epidermal, longitudinal muscle sheath of epitheliomuscular cells and a gastrodermal, circular sheath. In medusae and in members of the classes Scyphozoa and Anthozoa, the musculature has become separated from the epithelial tissue as a subepithelial system. In many anthozoans a large sphincter muscle develops in the gastrodermal surface, at the top of the column. In medusae, where the primary swimming muscles are subumbrellar, large muscle bands, both circular and radial, develop.
The mesogleal system is represented by a layer between epidermis and gastrodermis which varies from the thin structureless cementing layer of hydrozoan polyps to the highly complex cellular, fibrous, gelatinous matrix of the scyphomedusans and anthozoans. Where the mesoglea is well developed, as in medusae and some anthozoans, it serves as a type of internal skeleton, against which muscles may act.
Two different types of skeleton occur in the phylum. The first is of an internal nature and is either the mesoglea against which the muscles operate or the contained hydroskeleton of most polyps. Of a quite different nature is the exoskeleton, seen in most hydrozoan polyps, some scyphozoan polyps, and corals, which supports and protects the organisms. Still another skeletal type known in the coelenterates is an axial skeleton composed of sclerified proteins. It is exceedingly tough and flexible and is characteristic of such anthozoans as gorgonians.
The digestive system in coelenterates is developed as a function of the gastrodermis. In hydrozoans, both polyps and medusae, glandular, enzyme-producing cells are abundant throughout the gastrodermis. These cells secrete proteolytic enzymes into the coelenteron which reduce food objects to a fine particulate state. In scyphozoans and anthozoans, although glandular cells are common throughout the gastrodermis, they tend to be most abundant along the free edges of the mesenteries of anthozoans and on the gastric tentacles of scyphozoans. These glandular cells are also the source of proteolytic enzymes which act in the extracellular environment of the coelenteron.
The phylum is characterized by its carnivorous diet, made possible first by the possession of nematocysts which make the predaceous habit successful. After food has been trapped, movements of the tentacles carry it to the mouth, where with the help of ciliary and muscular devices the food is moved to the coelenteron. Here extracellular proteases prepare the way for final intracellular digestion. No herbivorous coelenterates are known.
The reproductive system of coelenterates consists of specialized areas of epithelia, the gonads, which periodically appear and produce gametes. There are no ducts for the sex products or any accessory sexual structures. Fertilization usually occurs in the water surrounding the animal, although a few coelenterates have their eggs fertilized in place and may then brood their young.
The ability to regenerate lost parts is characteristic of coelenterates. Pieces cut from almost any part of polyps will in time grow into new polyps. The regenerative powers of medusae are much less well developed, and not only will the excised piece not develop but it may not even be replaced by the medusa. Gradients of regenerative ability in polyps exist with the ability for a piece to reconstitute a new whole organism decreasing from the mouth to the base. See also Regeneration (biology).
The Coelenterata have a long and impressive fossil record stretching from the present into the Precambrian, about 700,000,000 years ago. Thus, the known duration of this phylum equals or exceeds that of other animal phyla. Forms with skeletons, primarily the Conulata, Tabulata, Rugosa, and Scleractinia, have left fairly complete fossil records, whereas soft-bodied forms, such as most of the Hydrozoa, Scyphomedusae, and Alcyonaria, are represented by very few fossils.
