THE WATER-MITES (HYDRACARINA) 



853 



or laterally, and in the males of certain species of Arrhenurus pro- 

 longed posteriorly into a curious handle-like appendage. The form 

 is more definite in the higher forms than in those which seem most 

 primitive. The skin in some forms is soft and the surface smooth, 

 but more usually it is marked by fine striae like the lines on the 

 palm of the hand, and in the lower forms it is often granulated or 

 papillated. Other species possess chitinous plates, which may be 

 few and small or larger and more numerous, and may even com- 

 pletely enclose the body in a sort of armor. These chitinous plates 

 do not seem to mark either higher or lower types and occur in 

 different famiHes. Glands occur here and there on the surface, and 

 also hairs and bristles, which are frequently accompanied by small 

 pieces of chitin. 



There is usually a pair of eyes, but each can be seen on close 

 examination to be double, and in some cases the two of each side 

 are separate. They are of only moderate size, but prominent, 

 owing to the presence of dark pigment. There may be also, in 

 some of the lower forms, a "fifth" 

 or median eye, in the median line 

 between the others. 



The four pairs of legs are artic- 

 ulated to an equal number of 

 coxal plates, or epimera. These are 

 frequently more or less fused, may 

 even form a single large plate cover- 

 ing the whole ventral surface, and 

 may also extend up on the sides so 

 as nearly to enclose the body, as in 

 Frontipoda. Sometimes the body is 

 constricted above this plate, giving 

 to the animal in lateral view the 

 appearance of a broad-crowned cap 

 or flat-based knob, the legs springing 

 from the upper side of the projecting 

 epimeral plate. The legs are each 

 composed of six segments, and vary greatly in length, in the 

 form of individual segments, and in the character of the spines, 



Fig. 1319. Pionacercus leuckarti Piersig, a 

 European form, showing extreme modi- 

 fication of the last pair of legs in the 

 male. (Legs shown on one side only; 

 palpi not shown.) (Modified from 

 Piersig.J 



