The coelenterates are among the simplest of the metazoans (multicellular organisms with cells organized in tissues). They include those animals in the phylum Cnidaria, such as corals, jellyfish, hydras and sea anemones, and the comb jellies of the phylum Ctenophora.
The largest species is the jellyfish Cyanea arctica measures 7.5 feet long while the smallest are the individual polyps of some coral colonies, which measure about 1 mm in length. Coelenterates comprise about 9,000 species and are probably the most common microscopic marine animals, especially in tropical and subtropical coastal waters. Furthermore, the corals create reefs and atolls, which are some of the world’s most diverse and productive habitats.
Cnidarians have two primary forms, polyps and medusae, which are radially symmetrical, with no definite front or rear ends. The flowerlike polyps are attached at their base to their mother organism and have a mouth on the upper side surrounded by tentacles (a long slender organ of touch or attachment). Medusae, commonly known as jellyfish, are free-swimming, bell-shaped organisms, with a mouth on the underside.
The coelenterate body comprises two layers of cells, an outer epidermis and an inner gastrodermis, separated by a layer of jelly-like matter: the mesogloea. The two layer of cells surround a central cavity (coelenteron) and contain muscle cells, which contract to bend the body or retract the tentacles.
In cnidarians, the epidermis contains stinging cells (nematocysts) to immobilize prey. The food is then pushed by the tentacles through the mouth into the coelenteron, where it is digested by enzymes and absorbed by the gastrodermis. Many coelenterates often form colonies of thousands of individuals.
Jellyfish (class Scyphozoa) consist of a swimming bell fringed by tentacles, with a four-cornered mouth on the underside. Around the mouth are four trailing arms which, like the tentacles, are well supplied with nematocytes.
Jellyfish swim by contracting and releasing a ring of muscle cells; balancing organs (statocysts) and simple light receptors around the bell margin help to remain upright. As their name suggests, they contain large amounts of the jelly-like mesogloea, which helps to control buoyancy, and also acts as an elastic support for the body.
In the typical life cycle of a cnidarian, the two basic forms alternate: polyps bud off medusae, which then produce a larval polyp, and so on. One form is usually dominant, and some species the other is omitted altogether.
The sea anemones and corals of class Anthozoa (flower animals) have a polyp stage only. These are often large and complex, with the coelenteron divided by partitions, or septa. Many sea anemones have large attachment disks and thick, leathery bodies, which allow them to survive on rocks which are exposed at low tide.
Corals are essentially colonial sea anemones, although solitary forms do exist. The polyps secrete skeletons, the exact form of which defines the species, such as the delicate sea fan (Gorgonia sp.) Reef-building corals flourish only in tropical and subtropical coastal waters where temperatures are above 72 degrees.
Ctenophores, or comb jellies, are grouped with cnidarians because they are also radially symmetrical, jelly-like, and composed of two layers of cells. These animals are marine and are usually found swimming among plankton; many are luminescent. They move using ciliated bands called comb plates. The cilia fuse into plates during the development of the animal; by beating these plates consecutively from the head to the tail, they swim through the water.
There are two classes: those with tentacles, such as the most common form, the sea gooseberry (Pleurobrachia pileus) and those without. Ctenophores catch their prey using lasso cells (colloblasts) which have a simple function to that of nematocysts in cnidarians, but they are adhesive rather than paralyzing.
The best-known hydrozoan, the Portuguese man-of-war (Physalia physalis) has a common hydrozoan feature, an air-filled cushion which acts as a float. Its tentacles comprise a colony of polymorphic polyps.
Some members of the class Hydrozoa, such as the common freshwater Hydra have no medusa stage although many other members of the class develop both polyps and medusae, and some display the medusoid form only. In the reproductive cycle of most hydrozoans, sexually reproductive medusoids bud off parents asexually, and either drifts free or remain attached to the parent organism. In the Hydra, however, reproduction is more commonly asexual.
Most hydrozoans are colonial and, although normally attached, move by a creeping action at the base. Most of them are green because of the presence of symbiotic algae (zoochlorella) in the gastrodermal cells.