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Phylum Nematoda Round worms |
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The Nematoda’s body has a shape like a string. A cross section of the body is round and hence called the round worms in general. The body is bilaterally symmetric, but only the mouth is uniquely shaped tri-radial symmetry. Certain species having tough teeth in the deep part of the mouth are fierce carnivorous predators. The body surface is covered with cuticles and shines like glittering glass. The Nematoda has no circulatory system. As a matter of course, nematodes are benthic animals in the sea; in addition, they can live on land where water is available. About 12,200 species of nematodes are known to us, but according to one estimate, about 100 million species of nematodes are yet to be described. The size of the animals varies greatly from few millimeters to one meter. In fertile soil, a record shows that 25,000 individuals of nematodes per square meter were inhabited. Many nematodes are free-living, and not far less number of species is parasitic. Among the species parasitic in plants and animals, pine wilt disease causing Bursaphelenchus xylophilus, trichinosis causing Trichinella spiralis, and filariae in dogs are very well known. Among the parasites that cause harms to humans, Ascaris lumbricoides and pinworms (Enterobius) are the representatives. Among the fossil record of the Nematoda, individuals trapped in an amber from the Carboniferous period (360 million ~ 290million years ago) are known. In general, there are two types of muscles in an animal’s body. Thanks to the movement of circular muscles on the outside and longitudinal muscles on the inside, animals can bend or stretch their body smoothly. However, the Nematoda only has the longitudinal muscles and cannot contract its body at all. One of the characteristic of this animal is that they can move by snapping action only. An individual nematode is either male or female (gonochroic), and the male is smaller than the female. Sexual reproduction is done by copulation, and the fertilization occurs in the female body. It is recently known that the sperms have a very peculiar shape. They lack flagella and have a system to live long in the female oviduct. Certain species of round worms keeps 27 million eggs in the body. Thus, they have superior reproductive ability. Unlike other invertebrates, round worms never transform their bodies in their entire lives. Thus, there are no peculiar-shaped larvae. In their life span, round worms molt four times in the course of the growth. Furthermore, at the certain stage of development, the cell division halts and only the size of each cell grows. A round worm called Caenorhabditis elegans is the first multi-cellular animal to have its genome completely sequenced, and established as a model lab animal. Developmental biology of C. elegans is also well-advanced, and the developmental fate of every region of embryonic cells is mapped out. In other words, the cell lineage is completely elucidated. It is well-known that the organ development and apoptosis of round worms are applied to researches on the human body and led to the Nobel Prize. |
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In preparation
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I am sorry.
A song is only Japanese.
Song
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karaoke
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Copyright (C); 2007, Shin Kubota.All Rights Reserved.