J. MURRAY ON WATER-BEARS, OR TARDIGRADA. 61 



name indicates, Muller regarded the Water-bears as mites, a view 

 of their relationship which is confirmed by the latest researches. 



After Midler's time a few naturalists regarded them as insects, 

 but for a long time the crustacean relationship was most 

 favoured. No good reason was ever given for uniting them with 

 insects or Crustacea, rather than with the mites, to some of the 

 lower genera of which they so closely approximate in outward 

 characters at least — the possession of four pairs of legs, and the 

 simple structure, without special organs for respiration or vessels 

 for the circulation of the blood. 



The recently discovered encystment of Tardigrada, which was 

 first noticed by Professor Lauterborn (13), otters a close parallel 

 to the process observed by Megnin and Michael in some of the 

 mites of the family Tyroghphidae (Cheese-mites, etc.), and 

 strengthens the belief in the affinity of the Tardigrada with the 

 Acari. 



Encystment. 



Many Water-bears of the three principal genera — Ecliiniscus, 

 Macrobiotus, and Diphascon — have been observed to form cysts. 



The process begins like an ordinary moult, but the animal 

 does not leave the old skin, but contracts within it till it forms 

 an elliptical body like an egg. This is the cyst, liow the outer 

 limbs are got rid of is not understood. 



Within the cyst, at any rate in the species which I have studied 

 most (20), a simplification of structure takes place, and the animal 

 loses most of its organs and all its hard parts (claw r s, teeth, 

 pharynx, etc.). The only recognisable parts remaining are the 

 eye-spots, the fat-cells, and some cells with dark contents in the 

 position occupied by the stomach. 



The regeneration of the organs has not been seen (though I 

 believe Professor Pichters is now studying the process), but I have 

 seen animals emerge from the cysts in a more or less complete 

 condition. They came out backwards, the posterior portion of 

 the case opening like a lid, remaining attached at one point. 



The process is strikingly like what Michael observed in the 

 mite Glycophacjus domesticus (see British Tyroylyphidae, Pay 

 Society, 1901, vol. i., pp. 168 — 173). There was a similar 

 liquefaction of the organs, loss of limbs, and the emergence was 

 likewise by a posterior trap-door. 



