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



In discriminating species, it must first be ascertained to which 

 group the species belong. 



Group I. — The plates are usually twelve in number (or eleven, 

 if segment v. is a single plate, instead of a pair), and are 

 arranged in the following sequence, beginning at the head : 

 Single (i.), single (n.), median (between it. and in.), pair (in.), 

 median (between in. and iv.), pair (iv.), median (between iv. 

 and v.), pair or single (v.), single, large, three-lobed (vi.) 

 (see Fig. 12). 



Any of the plates may-be subdivided ; the median are usually 

 divided into two by a transverse line. In the species figured 

 (E. mutabilis) segment n. is often obscurely paired, and the 

 median plates also. 



This group, which is thought by Richters to represent the 

 more primitive type of structure, includes only a few species, and 

 only three have been found in Britain. 



Group II. (Fig. 10). — The plates are nine (or ten if there is a 

 third median), and the arrangement is always that figured : 

 single (i.), single (n.), median, pair (m.), median, pair (iv.), 

 median (often absent), large three-lobed plate formed by the 

 fusing of v. and vi. 



This group contains the great majority of known species, and 

 these are distinguished chiefly by the various dorsal and lateral 

 processes. The commonest processes are five lateral (on each 

 side) and two dorsal, most of which are present on the species 

 figured. The lateral processes are denoted by the letters a, b, c, 

 d, e. In Fig. 10 the process e is absent ; when present it is at 

 the slit separating the lateral and median lobes of the large 

 plate (v. + vi.). The commonest dorsal processes are over the 

 lateral processes c and d. 



More than thirty species of Echiniscus have been described, 

 and eleven are recorded for Britain. 



Genus Echiniscoides Plate (25). 



Distinguished from Echiniscus by the more numerous claws — 

 seven to nine on each foot. The genus was founded by Plate as 

 a subgenus of Echiniscus. The only species, E. sigismitndi, was 

 described by Max Schultze as an Echiniscus in 1865 (43), and, 

 like Lydella, is marine. 



