The cnidarians and ctenophores are among the simplest of the multicellular organisms that have cells organized into tissues. Cnidarians comprise the phylum Cnidaria and include such animals as corals, jellyfish, hydras, and sea anemones. The largest species is the arctic jellyfish—7 feet (2.1 meters) across with tentacles 100 feet (30 meters) long—and the smallest are the individual polyps of some coral colonies, most of which are less than one inch (2.5 centimeters) in diameter. Most cnidarians are marine animals, although there are a few freshwater species. Scientists have identified about 10,000 species of cnidarians, and they are probably the most common macroscopic marine animals, especially in tropical and subtropical coastal waters.
Scientists have described about 100 species of ctenophores, which make up the phylum Ctenophora. All of them live in the sea and most drift about with the ocean current.
General structure of cnidarians
The cnidarians have two layers of cells that surround a tubular body cavity, with an opening at one end forming a mouth, and no specialized tissues for breathing or elimination of wastes.
Cnidarians have two basic forms polyps and medusas 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. Medusas, commonly known as jellyfish, are free-swimming, bell-shaped organisms, with a mouth on the underside. In the typical life cycle of a cnidarian, the two basic forms alternate-polyps bud off medusas, which then produce a larval polyp, and so on. One form is usually dominant, however, and in some species the other is omitted altogether.
The cnidarian body is made up of two layers of cells an outer epidermis and an inner gastrodermis-separated by a layer of jellylike matter called the mesogloea. The two layers of cells surround a central digestive cavity, or coelenteron, and contain muscle cells that contract to bend the body or draw in the tentacles. In cnidarians the epidermis contains stinging cells called nematocysts to immobilize prey. The tentacles then push the food through the mouth into the coelenteron, where it is digested by enzymes and absorbed by the gastrodermis.
Many cnidarians form colonies of thousands of individuals. In the simplest colonies all individuals are identical, but feed and reproduce separately. In more advanced colonies, however, such as those of some corals and hydro-zoans, several different forms are present, and each type undertakes a different function.
The class Hydrozoa has about 3,000, most of which live in salt water. Some hydrozoans, such as the common Hydra, live in fresh water. In most hydrozoans, symbiotic algae called zoochlorellae live in the gastrodermal cells of the body walls and give hydrozoans their characteristic green coloring.
Hydrozoans generally live as both medusas and polyps. Hydra and a few other species, however, have no medusa stage, and some hydrozoans display the medusoid form only. In the reproductive cycle of most hydrozoans, sexually reproductive medusoids bud off the parent and either drift free or remain attached to the parent organism, in which case they are known as gonophores. Hydra, however, more commonly reproduce by budding from the parent.
Most hydrozoans live in colonies of many individuals. Although they normally live attached to a surface, they can move by a creeping action at the base. Many hydrozoan colonies, such as the Portuguese man-of-war are attached to an air-filled cushion that acts as a float. Its tentacles are made up of a colony of polyps that is polymorphic, or made up of two or more types 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 nematocysts. Jellyfish swim by contracting and releasing a ring of muscle cells. Balancing organs called statocysts and simple light receptors around the edge of the bell help jellyfish remain upright. As their common name suggests, they contain large amounts of jellylike mesogloea, which helps to control buoyancy and also acts as an elastic support for the body.
Sea anemones and corals
The sea anemones and corals of the class An-thozoa (flower animals) have a polyp stage only. These polyps 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 that are exposed to the air at low tide.
Corals are essentially polymorphic colonial sea anemones, although solitary forms exist. The polyps secrete skeletons, the exact form of which defines the species, such as the delicate sea fan. Like hydrozoans, corals have photosynthetic algae called zooxanthellae living in their gastrodermal cells. Because of the algae’s need for light, reef-building corals flourish only in tropical and subtropical coastal waters where the water is less than 100 feet (30 meters) deep. These corals also require temperatures above 65° F.(18° C).
Ctenophores, or comb bearers, share several characteristics with cnidarians. They are radially symmetrical, jellylike, and composed of two layers of cells. But the cnidarians’ medu-soid shape has been adopted and modified into a sphere or oval. Ctenophores get their name from their comb plates, which form from fused cilia. There are two classes of ctenophores: those with tentacles, such as the most common form, the sea gooseberry, and those without. These animals are usually found drifting among plankton, and many are luminescent. They swim through the water by beating their comb plates consecutively from the head to the tail. Ctenophores catch their prey using sticky “lasso cells,” called colloblasts. Colloblasts have a similar function to that of nematocysts in cnidarians.