TARDIGRADA, a class of animals probably belonging to the Arthropoda and sometimes referred to the Arachnida (q.v.), but without good reason. The Bear-animalcules, as these animals were formerly called, are all of small size, the largest being not much over r mm. in length, while most of them are very much smaller. They are not parasitic but are found in very variable habitats, in damp moss, on flowering plants, in sand, fresh water and even in the sea; and in adaptation to this wide range of external conditions a very large number of genera and species have been evolved.
The organism consists of a well developed head-region in front and of a short body composed of four fused segments, each repre sented externally by a pair of short, stout, unjointed limbs gener ally terminated by sharp claws, which seem to range in number from four to nine, but sometimes, as in Batillipes, ending in three pairs of racquet-shaped, digitiform outgrowths. The limbs of the last pair project backwards from the posterior end of the body on each side of the anus. The cuticle, which is not chitinised, may be smooth or sculptured in various ways and is sometimes strengthened with segmentally arranged plates (Echiniscus) and sometimes provided with long paired hair-like cirri.
No special organs of circulation or respiration are known. The alimentary canal traverses the body from end to end. Within the mouth is a pair of protrusible stylets, large glands open into the oesophagus, there is a muscular pharynx and large median and lateral glands open into the intestine posteriorly. The nervous system, which is remarkably well-developed, consists of a bilobed cerebral ganglion, frequently provided with a pair of eye-spots, and of a ventral chain of five large ganglia connected by lateral commissures. The sexes are not distinct and the generative products are discharged either into the posterior end of the ali mentary canal or directly to the exterior through a median pore in front of the anus. The eggs are large and in some cases at least
are enclosed, when laid, in the cuticle of the parent which is cast to form a case for them. The young when hatched are about one third the size of the parent which they closely resemble apart from the occasional absence of one pair of limbs which is subse quently developed.
The Tardigrada are di Aded into two orders:—(i) The Eutardi grada (Macrobiotus, Hypsibius, Milnesium) in which the head is without cirri, the legs are not telescopically retractile and the four claws of each foot are coalesced to form a single pair of branched claws ; and (2) the Heterotardigrada (Echiniscus, Batil lipes, etc.) in which the head is provided anteriorly with two pairs of cirri, and on each side with one long cirrus and a shorter spatu late process, the clava, the legs are to a varying extent telescopi cally retractile and the claws are not coalesced.
The Tardigrada cannot be assigned with complete confidence to the Arthropoda, or Gnathopoda, until it be proved that their oral stylets are modified appendages. Assuming that they belong to the Arthropoda, they must rank as a class by themselves. They have been generally relegated to the Arachnida on the supposition of their kinship with the Acari, but their internal and external anatomy make such an affiliation unlikely. They have also been compared to Peripatus mainly on account of the structure of their limbs ; but there seem to be no good reasons for regarding the two types as related. (R. I. P.) TARDIVEAU, RENE: see BOYLESVE, RENT.