268 W, SAVILLE-KEXT, CONTRIBUTIONS TO A 



body. These chitinous expansions, the epimera, usually forming 

 two independent symmetrical lateral series, but occasionally 

 continuing with one another in the median line, while morpho- 

 logically homologous with the motile basal elements of the seven- 

 jointed ambulatory limbs of the true spiders, exhibit a more 

 strict physiological correspondence with the expanded adnate 

 bases of attachment or sternal elements as developed in the 

 scorpions. The comparative proportions and the positions rela- 

 tive to each other occupied by the epimera of the Hydrachnidae 

 are found to afford valuable correlative data for the construction 

 of their generic and specific diagnoses. In most instances the- 

 six-jointed locomotive appendages dependent from the epimera 

 terminate similarly in a double or yet more complex claw, 

 accompanied frequently by a groove-like excavation of the 

 terminal joint of the limb from which it extends, and into 

 which groove it may be reflexed at w^ll as into a sheath. In 

 some forms the claw-like structure is trifid or even quadrifid, 

 while, on the other hand, in certain genera, e.g. Limnesia 

 Koch, the hindermost limb has a perfectly simple non-ungulate 

 termination. It is worthy of notice that in not a few genera,, 

 such as Nesaea, Arrheaurus and Hydrochorentes^ the ante- 

 penultimate joint of the third or fourth locomotive limb in the 

 males only is so modified as to constitute an efficient crook or 

 clasper, for the better retention of the female during copulation. 

 A similar structural and functional modilication of the hinder 

 limbs in the male individuals is found to obtain among many 

 of the terrestrial Acari, notably in the parasitic genus Derma- 

 leickas, but does not seem to occur among any of the higher 

 Arachnida. 



While the fourth pair of legs in the Hydrachnidae represents, 

 as in all other Arachnida, the most posteriorly developed jointed 

 appendages, there yet exist what it is here anticipated may be 

 correctly interpreted as representing the sternal elements or 

 epimera of the succeeding body somite. On the ventral surface 

 of these Water-mites, in fact, immediately behind or at some 

 little distance from the epimera of the fourth pair of locomotive 

 limbs and between these latter and the anal aperture, will be 

 generally found two adnate bilaterally symmetrical chitinous 

 plates, of various forms and ornamentations in different species^, 

 which enclose between them the genital fissure. Although such 



