62 



J. MURRAY ON WATER-BEARS, OR TARDIGRADA. 



The mite has a well-marked metamorphosis, and the retrogres- 

 sion by which the cyst is produced takes place in one of the 

 immature stages, and undoubtedly facilitates the distribution of 

 the species. The Water-bear has no metamorphosis, and the 

 cysts appear to be formed after maturity is reached, and do not 

 subserve distribution. It is probably in this case a protection 

 during unfavourable conditions. 



The degeneration of the organs during encystment may throw 

 some light on the so-called simplex forms. These may be 

 animals which are losing or regaining the organs in connection 

 with encystment, but as simplex forms exist even in the egg, we 

 cannot regard this as a sufficient explanation. 



Key to the Genera of Tardigrada. 



1. One claw on each foot 

 Several claws on each foot 



2. Back armour-plated . 

 Back not armour-plated 



3. Claws four, in larva two 

 Claws seven to nine (usually eight) 



4. Palps on head, no rods in pharynx 

 No palps on head, rods in pharynx 



5. An elongate flexible gullet . 

 Gullet not elongate and flexible . 



Lydella. 

 . 2 

 . 3 



Echiniscus. 

 Ech iniscoides. 



Milnesium. 

 . 5 



Diphascon. 

 Macrobiotus. 



Genus Macrobiotus Schultze (40). 



The animals of this genus are narrow and elongate, soft, and 

 obscurely segmented, with no dorsal shields and no palps or 

 bristles on the head. There are four claws on each foot, and 

 they are always associated in two pairs, laxly or firmly united 

 (see Figs. 1, 2, 4, and 9). The claws of each pair are more or 

 less unequal, and the longer one has a bristle or supplementary 

 point behind the principal ,point (Figs. 7 and 8), or two such 

 points (Fig. 9). 



The four principal varieties of claws are figured : (1) the 

 claws join at or near the base only, each pair forming a V with 

 slightly unequal legs, and the two pairs similar (Fig. 2) ; (2) claws 



