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.
Hydrozoans
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.
Jellyfish
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.
Ctenophores
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.
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