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TARDIGRADA

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Originally appearing in Volume V26, Page 418 of the 1911 Encyclopedia Britannica.
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TARDIGRADA , apparently Arthropodous animals whose relationship to the See also:

great classes of this sub-See also:kingdom is masked by degenerative modification. They are microscopical in See also:size and live in See also:damp See also:moss or See also:water. The See also:body is elongated and furnished with four pairs of See also: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 See also:oesophagus, into which the ducts of a pair of so-called salivary glands open. Behind this point there is a See also:muscular pharynx or gizzard, which communicates with the wide intestinal See also:tract. No See also:organs of circulation or respiration are known; but the See also:nervous See also:system is well See also:developed, and consists of a pair of ganglia corresponding with the limbs and connected by See also:longitudinal commissural chords. Anteriorly these chords embrace the oesophagus and unite with the cerebral See also:mass which innervates the pair of eyes when See also: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 See also:canal. The Tardigrada have been regarded as degenerate Acari largely on See also:account of their possessing four pairs of See also:ambulatory limbs, which is considered II Milnesium tardigradum, Schrank. a, ovary; b, See also:oval stylite (?) ; c, mouth; d., alimentary canal; e...e, legs. to be an Arachnidan characteristic. But they cannot be affiliated with this See also:order on account of the See also:total suppression of the See also:abdomen, of their hermaphroditism and of the communication that exists between the generative organs and the alimentary tract.

These last characteristics also See also:

separate them essentially from the See also:Pycnogonida, some members of which resemble them to a certain extent in having only four pairs of limbs, no gnathites, no See also:respiratory organs, a ganglionated ventral nervous system, and the abdomen reduced to a See also:mere rudiment projecting between the last pair of legs. Several genera and See also:species of Tardigrada have been described, perhaps the best known being Macrobiotus schultzii and Milnesium tardigradum. (R. I.

End of Article: TARDIGRADA

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