Cnidaria

Cnidaria is one of the more primitive animal phyla. It includes aquatic organisms such as jellyfish, sea anemones, corals, and hydras. Most cnidarians are marine, although a few, such as the well-known hydra, are freshwater species.

Characteristics of Cnidarians

All cnidarians are characterized by radially symmetric body plans, rather than the bilaterally symmetric body plans that are found in most other animal phyla. Although cnidarians are more advanced than sponges (phylum Porifera) in that they possess distinct tissue layers, they lack many of the features of more advanced animal phyla, such as internal organs and central nervous systems. Most cnidarians possess tentacles, and many also have nematocysts (specialized stinging cells). Both are involved in feeding.

Cnidarians are characterized by the presence of three tissue layers, an outer protective epidermis, a middle layer called the mesoglea, and an inner layer called the gastrodermis, whose function is primarily digestive. The mesoglea of cnidarians is not as highly developed as the mesoderm of other animal groups, being primarily gelatinous with only a few fibrous or amoeba-like cells.

Cnidarians possess only one digestive opening, which serves as both the mouth and the anus. This opening is surrounded by tentacles and leads to an internal digestive cavity called the gastrovascular cavity .

Cnidarians feed using tentacles that are embedded with stinging nematocysts. The nematocysts are springing barbs with small hairlike triggers that are activated by contact with prey. Most nematocysts require stimulation in more than one sensory mode before they will fire. For example, a nematocyst may respond only if there is mechanical stimulation from physical contact with the prey as well as chemical stimulation signaling the presence of suitable prey. As nematocysts fire, barbs unfold and become embedded in the tissue of the prey. At the same time, the nematocysts inject the prey with an immobilizing toxin through a long hollow thread within the barb. Once the prey item has been captured and subdued, tentacles are used by the cnidarians to bring the prey item into the gastrovascular cavity. Within the gastrovascular cavity, the food item is broken into small particles by digestive enzymes secreted by gastrodermal cells lining the cavity. The minute particles are then taken in by the gastrodermal cells, and digestion is completed in digestive vacuoles (small cavities) within these cells. The indigestible remnants of the prey are expelled from the mouth of the gastrovascular cavity.

One hypothesis about the origin of nematocysts suggests that they were prokaryotic endosymbionts which lived within eukaryotic cells as mutualists (mutualisms are symbiotic relationships between individuals of two different species, in which members of both species derive benefits from the relationship), the same way organelles (specialized parts of cells) such as mitochondria and chloroplasts are believed to originate.

Unlike more advanced animal phyla, cnidarians lack a central nervous system. Instead, their nerves are organized in nerve nets that cover the entire body. Impulses spread slowly out from the point of stimulation along the nerve net. Some cnidarians, such as jellyfish, have more complicated arrangements of nerves that allow for more complex responses to stimuli as well as more effective patterns of movement.

Cnidarians also lack certain tissue types found in other animal phyla, such as true muscle cells. However, they do have fibers that can contract and therefore can be used in capturing prey and in moving about.

Major Groups of Cnidarians

Cnidarians are divided into three major classes. These are the Hydrozoa (hydras and other colony-forming species), the Scyphozoa (jellyfish), and the Anthozoa (sea anemones and corals).

Hydrozoa.

The best-known member of the Hydrozoa is the hydra, a freshwater species. However, the hydra is not a typical hydrozoan. For example, the hydra has only a polyp stage, for example, whereas most hydrozoans have a biphasic (two-stage) life cycle that alternates between a sedentary polyp stage and a mobile, bell-shaped medusa stage. The hydra is not strictly sedentary; it moves in a very unusual way, by turning somersaults. In addition, most hydrozoans are colonial, with each colony arising from the asexual budding of a single individual. Members of a hydrozoan colony have interconnected gastrovascular cavities, and the fluid in this cavity is circulated by cells with long, beating flagella . There is typically some degree of division of labor within the colony. Usually, there are feeding polyps, which possess tentacles and nematocysts (stinging cells), and reproductive polyps, which continually bud off tiny mobile medusas. The medusas swim by tightening and relaxing cells within the bell, and are also scattered by prevailing water currents. Medusas release sperm and eggs directly into the water, where fertilization occurs. The zygote (fertilized egg) develops into what is called a planula larva—the larvae of cnidarians. The larva ultimately settles to the substrate (rocky bottom of the ocean), finds something to anchor to, develops a mouth and tentacles, and becomes a polyp that subsequently buds to form a new colony.

Scyphozoa.

The Scyphozoa includes the well-known jellyfish. In this group, the polyp stage is far less significant than among the Hydrozoa, since the medusa stage is dominant. Scyphozoan medusas grow to sizes considerably larger than those found among the Hydrozoa. They range in size from a few centimeters to over 2 meters across. The nervous systems of jellyfish are also more developed than those of other cnidarians. Instead of a simple nerve net, they have a nerve ring around the edge of the bell portion of the medusa. Neurons throughout the rest of the body connect to this ring. This organization allows for faster conduction of impulses from one side of the body to the other, which in turn allows the jellyfish to swim with coordinated contractions of the entire bell.

Anthozoa.

The Anthozoa includes the sea anemones and the corals. These species lack the medusa stage altogether, and exist exclusively in the polyp form. Anthozoans tend to have more highly developed contractile cells (cells capable of contracting) than other cnidarians, as well as a more highly developed, thicker mesoglea, which often forms a fibrous connective tissue. Corals secrete a hard, limy skeleton and can form huge reefs, such as the Great Barrier Reef off the coast of Australia. Coral reefs are an impressive ecosystem, one of the most diverse and productive on Earth.

see also Phylogenetic Relationships of Major Groups.

Jennifer Yeh

Bibliography

Anderson, Donald Thomas. Atlas of Invertebrate Anatomy. Sydney, Australia: UNSW Press, 1996.

Barrington, Ernest James William. Invertebrate Structure and Function. New York: Wiley, 1979.

Chesher, Richard H. Living Corals. New York: C. N. Potter, 1979.

Clarkson, Euan Neilson Kerr. Invertebrate Paleontology and Evolution. Boston: G. Allen and Unwin, 1979.

Mayer, Alfred Goldsborough. Medusae of the World. Washington, D.C.: Carnegie Institution of Washington, 1910.

Cnidarian

A cnidarian (ny-DAIR-ee-uhn) is a simple invertebrate (an animal without a backbone) that lives in the water and has a digestive cavity with only one opening. Jellyfish, sea anemones, corals, and hydra are all cnidarians. Cnidarians catch food using armlike, stinging tentacles and their bodies have only two tissue layers, unlike higher animals who have three layers.

While a cnidarian may be more complex than a sponge, it is still a very simple invertebrate. Also called a coelenterate, which means "hollow gut," a cnidarian has been described as having a body that resembles a sack or a hollow bag with a hole in it. This hole is its baglike digestive cavity that has a single mouth or opening at one end. Both food and waste pass through this opening which is usually surrounded by armlike extensions called tentacles. These tentacles are equipped with stinging cells called cnidoblasts that shoot poisonous, spiny threads. Cnidarians use these threads to paralyze and capture small animals that swim into them. The unmoving fish is then pushed by the tentacles through the mouth and into the digestive cavity. Once there, it is broken down and eventually absorbed by cells lining the cavity. There are three main groups of cnidarians: the hydra, the jellyfish, and the corals.

The simplest cnidarian is the hydra, and it is one of the few freshwater cnidarians. Although it is especially tiny, resembling a piece of string on a pond, under a microscope the hydra reveals a hollow trunk with a mouth at one end that is surrounded by a ring of tentacles. It captures tiny animals by stinging them and can stretch when reaching for food. A hydra can reproduce asexually by budding (growing new cells that separate from the parent) or by regeneration (growing a complete organism from a piece).

The jellyfish belongs to a class whose name translates as "cup animals." As an adult, it has an umbrella-shaped or bell-shaped body and swims by pumping water into and out of its digestive cavity. Its mouth is in the center of its underside and has long mouth lobes that hang down from it. The nearly colorless jellyfish we often see while swimming is probably the moon jellyfish. All jellyfish reproduce sexually, joining sperm and egg to produce a larva that first swims around and then attaches itself to the bottom and grows as a polyp. It then breaks off and swims away to assume the shape called a medusa. Since a jellyfish's body is mostly water, when it dies and is washed ashore, it soon dries and leaves only what appears to be a circle of film.

Some other cnidarians do not have the freedom to swim about and spend their entire lives attached to something at the bottom of the ocean, usually with many other of their kind in what are called colonies. These are the coral and sea anemones. Both have tubelike bodies and resemble what some call flower animals. They are brightly colored and gather in clusters that resemble blossoms. Most corals are very small and protect themselves by building hard cases of calcium carbonate (limestone) around themselves. During the day, corals hide inside their shell, but at night they extend their tentacles and catch tiny animals swimming by. When a coral dies, its limestone covering remains and coral reefs are built, formed by countless covering after covering. One large example of a coral reef that has developed over time is The Great Barrier Reef off the coast of Queensland, Australia, which is 95 miles (152.9 kilometers) long.

Cnidaria A phylum of aquatic invertebrates (sometimes known as coelenterates) that includes Hydra, jellyfish, sea anemones, and corals. A cnidarian's body is diploblastic, with two cell layers of the body wall separated by mesoglea, and shows radial symmetry. The body cavity (gastrovascular cavity) is sac-shaped, with one opening acting as both mouth and anus. This opening is surrounded by tentacles bearing thread cells. Cnidarians exist both as free-swimming medusae (e.g. jellyfish) and as sedentary polyps. The latter may be colonial (e.g. corals) or solitary (e.g. sea anemones and Hydra). In many cnidarians the life cycle alternates between these two forms (see alternation of generations). The phylum contains the classes Hydrozoa (e.g. Hydra, Obelia), most members of which show alternation of generations; Scyphozoa (jellyfish), in which the medusa phase is dominant; and Anthozoa (corals and sea anemones), in which medusae are absent. See also coelenterate; Ctenophora.

Cnidaria (Coelenterata) Phylum comprising the sea anemones, jellyfish, and corals, which is known from the late Precambrian. The Ediacaran fauna from southern Australia, from between 680 and 580 Ma ago, includes clearly identifiable jellyfish and their allies. Corals occur for the first time in Ordovician rocks. Cnidarians are all aquatic and mostly marine. The body plan is characterized by a single internal cavity, the enteron (or gut), and the body wall is composed of two layers of cells separated by a gelatinous mass (the mesogloea). There are two basic body shapes, the sessile polyp (e.g. sea anemones, corals) or the free-swimming medusa (e.g. jellyfish). The mouth is surrounded by tentacles armed with stinging cells (nematocysts) that are unique to cnidarians. Typically, the body approaches radial symmetry, though some forms are bilaterally symmetrical. See ANTHOZOA; HYDROZOA; and SCYPHOZOA.

Cnidarian

The Cnidaria (pronounced ny-DARE-ee-ah) are a phylum of simple animals including the hydras, jellyfish, sea anemones, and corals. Any swimmer who has suffered a jellyfish sting has painfully encountered the feature for which the phylum is named: the venomous, stinging organelles called nematocysts or cnidae (pronounced NID-ee). Nematocysts are used for defense and to sting and paralyze prey, ranging from plankton to fish.

Cnidarians have a simple body plan with two epithelial cell layers: the epidermis and gastrodermis, separated by a gelatinous mesoglea ("middle glue"). The mesoglea ranges from a thin, gluelike layer in the freshwater hydras to a thick, gelatinous layer in the jellyfish. The simple body wall encloses a water-filled space, the gastrovascular cavity, responsible for the digestion of food and the distribution of digested nutrients.

Many cnidaria have a life cycle that alternates between a sessile polyp stage and a swimming medusa. The polyp may consist of a single stalklike body, attached to the substrate below and with a mouth surrounded by a ring of tentacles above; or it may be a branching colony, easily mistaken for a plant until one looks at it under the microscope. The medusa (jellyfish) is typically umbrella shaped, with a mouth-bearing stalk where the handle of the umbrella would be, and stinging, nematocyst-laden tentacles around the margin. Hydras, corals, and sea anemones, however, have only the hydroid stage, and some medusae have no polyp stage in the life cycle.

see also Animalia; Coral Reef; Ocean Ecosystems; Plankton

Kenneth S. Saladin

Bibliography

Pechenik, Jan A. Biology of the Invertebrates, 4th ed. Boston: McGraw-Hill, 2000.

Rupert, Edward E., and Robert D. Barnes. Invertebrate Zoology, 6th ed. Forth Worth, TX: Saunders College Publishing, 1994.