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Encyclopedia Britannica

TARDIGRADA

This article appears in Volume V26, Page 418 of the Encyclopedia Britannica.

Encyclopedia Britannica - Main :: SUS-TAV

TARDIGRADA , apparently Arthropodous animals whose relationship to the

great

GREAT

classes of this sub-kingdom is masked by degenerative modification. They are microscopical in size and live in

damp

DAMP

moss

MOSS

or water. The

body

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

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

TRACT (from Lat. tractare, to treat of a matter, through Provencal tractat and Ital. trattato)

. No organs of circulation or respiration are known; but the nervous

system

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

OVAL (Lat. ovum, egg)

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

TOTAL

suppression of the

abdomen

ABDOMEN (a Latin word, either from abdere, to hide, or from a form adipomen, from adeps, fat)

, of their hermaphroditism and of the communication that exists between the generative organs and the alimentary

tract

TRACT (from Lat. tractare, to treat of a matter, through Provencal tractat and Ital. trattato)

. These last characteristics also

separate

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

SYSTEM

, and the

abdomen

ABDOMEN (a Latin word, either from abdere, to hide, or from a form adipomen, from adeps, fat)

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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