Cnidarians - biology.

Cnidarians - biology. I INTRODUCTION Cnidarians, also known as coelenterates, diverse group of aquatic, invertebrate animals armed with microscopic stinging structures. Cnidarians make up the phylum Cnidaria, which encompasses more than 9,000 species, including corals, hydras, jellyfish, Portuguese man-of-war, and sea anemones. Cnidarians live in all oceans, and a few species inhabit fresh water. II PHYSICAL DESCRIPTION Cnidarians have many shapes and they range in size from microscopic hydrozoans to jellyfish that are 2 m (7 ft) in diameter with tentacles 30 m (100 ft) long. Although they have various physical characteristics, all cnidarians exhibit radial symmetry--that is, similar body parts radiate from a central mouth. Six to ten tentacles surround a cnidarian's mouth to aid in the capture and ingestion of the animals they feed on. Cnidarians have a saclike body with a single mouth opening. The body wall is composed of two sheets of cells--an inner layer (the endoderm) and an outer layer (the ectoderm). A gelatinous mesoglea layer holds these two cell layers together. Cnidarians are invertebrates (animals that lack a backbone), but the ectoderm of some cnidarians, including hard corals and some hydrozoans, may form a skeleton-like structure externally. The ectoderm of other cnidarians, such as some soft corals, forms an internal skeleton-like structure. The ectoderm and endoderm layers contain contractile fibers that enable the animal to move about. Invertebrate zoologists believe these fibers are primitive versions of the muscle cells found in more complex animals. Cnidarians lack internal organs and they do not have digestive, circulatory, or respiratory systems. Secretions from endoderm cells digest food within the central body cavity and endoderm cells also distribute nutrients and dissolved oxygen to all parts of the body. Lacking an anus, cnidarians discharge waste matter through the mouth opening. A Polyp and Medusa Structures During their life cycle cnidarians may form two types of body structures, the polyp and the medusa. Some cnidarians alternate between polyp and medusa at different stages of their life cycles, some cnidarians live solely as polyps, and others only as medusae. A polyp has a cylindrical shape. The mouth, surrounded by tentacles, is located at one end of the polyp, and the opposite end of the polyp typically attaches to a rock or other object on the sea bottom. Polyps may be solitary, as in some types of hydras, or they may attach to other polyps to form a colony, as corals do. In some cnidarians, such as those from the genus Obelia, the polyps of a single colony may perform specialized functions. Some polyps in a colony capture food, others reproduce, and others protect the colony. The medusa body resembles a saucer or umbrella. The mouth is located in the center of the undersurface of the saucer-like body, surrounded by tentacles. Medusae are solitary and most swim freely in the water. The gelatinous mesoglea layer in medusae is quite thick, which is why cnidarians with the medusae body form are commonly referred to as jellyfish. B Nematocysts The name cnidarian derives from the Greek word knidos, meaning "nettle," referring to the microscopic stinging structures known as nematocysts found on cnidarian tentacles. A cnidarian will stun a prey animal with its nematocysts and then use its tentacles to bring the prey to its mouth to swallow. Each nematocyst contains a coiled tubule that shoots out like a harpoon with explosive force. There are about 30 kinds of nematocysts that work in different ways to stun prey. The tubules of some nematocysts wrap around prey. The tubules of other nematocysts stick to prey, while other tubules pierce the surface of small animals. The nematocysts of some cnidarians contain poison that can paralyze or kill prey. III REPRODUCTION Reproduction in cnidarians varies among the different species. They may reproduce by means of asexual reproduction, sexual reproduction, or both. Polyps generally perform asexual reproduction by budding, in which an outgrowth from the body wall separates to form a new polyp or medusa. Medusae primarily reproduce sexually--they produce gametes (sex cells), and a gamete (sperm) from a male medusa fuses with a gamete (egg) from a female medusa to form a zygote. The zygote develops into a larva, which in turn develops into a polyp or medusa. The medusae of some cnidarians may also form polyps by budding. The reproductive life cycle of a typical jellyfish illustrates both asexual and sexual reproduction. Males release sperm and females release eggs into the water. When an egg and sperm fuse during sexual reproduction, a larva develops that attaches to a rock or other object and develops into a polyp. In a type of asexual reproduction, the polyp divides to form a colony of polyps that resembles a stack of saucers. Each saucer in the stack develops tentacles and swims away from the colony as a new medusa, and the reproductive cycle repeats. IV TYPES OF CNIDARIANS Scientists divide cnidarians into four classes: Hydrozoa, Scyphozoa, Cubozoa, and Anthozoa. They base this division partly on whether the polyp or medusa is more conspicuous during an animal's life cycle. A Hydrozoa In Hydrozoa, the polyp phase dominates in the animal's life cycle. Most hydrozoan polyps reproduce by budding to form a polyp colony that fastens to solid objects such as rocks and pilings. Some hydrozoan polyps also bud to form a medusa, which is generally small and lives only a short time. This class comprises about 3,100 species, including the freshwater hydra (which lacks a medusa stage), the Portuguese man-of-war, and fire coral. B Scyphozoa The medusa phase dominates in most of the 200 species of scyphozoans, the cnidarians most frequently referred to as jellyfish. Scyphozoan medusae are larger and have a more complicated structure than the short-lived medusae of the hydrozoans. Scyphozoan medusae live several months or more, reproducing sexually to form small, inconspicuous polyps. Polyps reproduce asexually to form a medusa--saucer-like structures bud off the polyp and swim away as new medusae. C Cubozoa The medusa phase dominates in animals in the Cubozoan class. Cubozoan medusae are commonly called box jellies because they have a cube shape with a single tentacle or group of tentacles hanging from each corner at the mouth end of the animal. The Cubozoan polyp is small and inconspicuous and an entire polyp transforms into a medusa in a type of asexual reproduction. Cubozoa consists of about 20 species of sea wasps and their relatives. The nematocyst sting of some cubozoans, including the Australian sea wasp Chironex fleckeri, contains poison that can be fatal to humans. D Anthozoa In the Anthozoa class there is no medusa stage. An anthozoan polyp reproduces sexually, although the polyps of some species also reproduce asexually. Asexual reproduction results in a colony if the daughter polyps remain attached to one another. There are about 6,500 species of anthozoans, including corals, sea anemones, sea pens, and sea fans (see Gorgonian). Scientific classification: The phylum Cnidaria is made up of the classes Hydrozoa (predominately polyps), Scyphozoa (primarily medusae), Cubozoa (primarily medusae), and Anthozoa (polyps only). Reviewed By: Daphne G. Fautin Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. 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