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Phylum Cnidaria Jellyfish, corals, and sea anemones Lyrics: Shin Kubota, Music, Vocal: Beni-Kurage Music and Arragement: Sillty |
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About ten thousand species of animals belong to Cnidaria. They include jellyfish, corals, sea anemone and others, and most of them live in the sea. Fresh-water species such as Craspedota, which occasionally make the headline of newspapers by suddenly appearing in a pond, is a minority group. Their body sizes vary greatly. Hydrozoans can be less than 1mm, and the Lion’s mane jellyfish is the longest animal on the planet with tentacles about 40 meters in length. Nomura’s jellyfish weigh over 200 kg and its bell reaches 2 meters in diameter. Off the coast of Japan, many Nomura’s jellyfish were accidentally caught in fishing nets and cause damages to fish and fishing gears. A very special characteristic of Cnidaria is the mysterious power of immortality. A tiny hydrozoa Turriopsis with a bell diameter of about 1 cm has such ability. When the surrounding environment becomes deteriorated, it transforms into a juvenile polyp in several days, and the rejuvenation can be repeated many times. In contrast to any other animals which die after producing offspring, this miracle animal has gotten much attention from TV, radio, and journals. The body structure of cnidarians is rather simple, but unlike Porifera, cnidarians have nerve fibers, sensory organs and muscles, and in a sense, they look like real animals. The body surface is covered with special epithelial muscular cells which are skins as well as muscles. The muscle of sea anemone and large jellyfish is large enough to see with the naked eye. Basically, both the medusa and the polyp are “stomach animals” that have elastic bodies and can capture preys larger than themselves. In the gastrula stage, the mouth also has the function of anus, and it is diploblastic having outer ectoderm and inner endoderm. Characteristics of Cnidaria are that a fertilization membrane does not move up and a fertilized egg cleaves at one end of the egg and the shape of the cell cleavage looks like a heart. Cnidarians have high capacity of regeneration. If a tentacle is severed, it can be regenerated in a day. If a part of its body is bitten off by a predator, the missing part will be regenerated in a short period of time. If you grind the polyp down into tiny pieces, cells will gather to form an aggregate, and it eventually becomes a polyp. The process is very similar to that of sponge. Siphonophora and colonial corals have developed division of labor. For example, one section is specialized in capturing food, another section is for attacking or defending, and the rest of the section is for feeding. This type of life strategy is similar to the society of ants or bees. There are more than 30 different types of stinging cells called cnida (nematocyst) which is the origin of this phylum’s name. Different taxonomic groups may have different types of nematocyst. Thus, the nematocyst may be used as a key to classify different taxonomic groups. The nematocyst is stored in a cell as a cellular organelle. It is mainly stored in ectoderms such as tentacles. When stimulated by certain cues, the coiled thin long tube (stinging thread) discharge promptly as it everts at high speed. It is mechanically structured such a fine way that the tube penetrates deep into the foreign body and injects venomous substance from the tip of the tube. Stinging effect of Portuguese man-of-war and other jellyfish such as Stephanoscyphus racemosus can be very strong that some people experienced painful swelling by the sting. The sea wasp jellyfish in tropical water is much more venomous than the habu viper jellyfish Chiropsalmus in Okinawa and people need to be careful because the sting may cause human deaths. |
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Copyright (C); 2007, Shin Kubota.All Rights Reserved.