Online Encyclopedia

TARDIGRADA

Online Encyclopedia
Originally appearing in Volume V26, Page 418 of the 1911 Encyclopedia Britannica.
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TARDIGRADA, apparently Arthropodous animals whose relationship to the great classes of this sub-kingdom is masked by degenerative modification. They are microscopical in size and live in damp moss or water. The body is elongated and furnished with four pairs of short, unjointed, stump-like legs, each terminated by a pair of claws. The legs of the posterior pair project from the hinder extremity of the body and the anus opens between them. The mouth, situated at the opposite end and armed with a pair of stylets, leads into an oesophagus, into which the ducts of a pair of so-called salivary glands open. Behind this point there is a muscular pharynx or gizzard, which communicates with the wide intestinal tract. No organs of circulation or respiration are known; but the nervous system is well developed, and consists of a pair of ganglia corresponding with the limbs and connected by longitudinal commissural chords. Anteriorly these chords embrace the oesophagus and unite with the cerebral mass which innervates the pair of eyes when present. The sexes are not distinct, the sexual organs being represented by a pair of testes and a single ovary, which open together into the posterior end of the alimentary canal. The Tardigrada have been regarded as degenerate Acari largely on account of their possessing four pairs of ambulatory limbs, which is considered II Milnesium tardigradum, Schrank. a, ovary; b, oval stylite (?) ; c, mouth; d., alimentary canal; e...e, legs. to be an Arachnidan characteristic. But they cannot be affiliated with this order on account of the total suppression of the abdomen, of their hermaphroditism and of the communication that exists between the generative organs and the alimentary tract. These last characteristics also separate them essentially from the Pycnogonida, some members of which resemble them to a certain extent in having only four pairs of limbs, no gnathites, no respiratory organs, a ganglionated ventral nervous system, and the abdomen reduced to a mere rudiment projecting between the last pair of legs. Several genera and species of Tardigrada have been described, perhaps the best known being Macrobiotus schultzii and Milnesium tardigradum. (R. I. P.)
End of Article: TARDIGRADA
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