262 W. SAVILLE-KENT, CONTRIBUTIONS TO A 



only the shapes but for the most part the original brilliant 

 tints of the little animals are perfectly retained, has been of the 

 utmost service in the compilation of this memoir. Before pro- 

 ceeding to technicalities it is desirable to mention that all of the 

 examples upon which these observations are based were collected 

 by me in the neighbourhood of London, and number approxi- 

 mately, with additions made during the past autumn, 1882,. 

 upwards of eighty species. 



General Morphology 



As members of the class Arachnida the Hydrachnidae or 

 Water-mites, as their name implies, constitute a subdivision 

 of the typical mites or Acarina, specially adapted by the garni- 

 ture of their locomotive appendages for leading not only an 

 aquatic but an essentially natatory existence. Such garniture 

 takes the form of long slender hairs or setae, developed for 

 the most part in bundles upon the internodes of the ambulatory 

 limbs, and with the aid of which the little creatures propel them- 

 selves through the water with remarkable activity. But for the 

 possession of these slender swimming-setae the Hydrachnidae 

 correspond essentially in all broad structural details with the 

 ordinary mites or Acarina, and types are not wanting that serve 

 to indicate the close connection that subsists between these respec- 

 tive groups. The genus Ealais of Duges,* in w^hich the posterior 

 pair of limbs is entirely devoid of swimming-setae and takes 

 no part in natation, may be cited among the swimming forms 

 belonging to the Hydrachnidae, while the aquatic but altogether 

 non-natatory genera Atractides and Thyas of C L. Koch assist 

 to liridge the two groups from the opposite side. As a 

 character, albeit of a negative order, that serves to distinguish 

 the Hydrachnidae from the more ordinary terrestrial Acaridae 

 it is further found that in no form as at present known is the 

 body suVjdivided by transverse constrictions into distinct regions 

 or somites as obtains in certain genera such as Sciurus, Kupodes 

 nnd TyroglyphujS^ the whole mass corresponding with the head 



* The reference for the genus Eulais or Ei/lais is Latieille 17i)(), nut 

 Duges 18o4. At the time of Saville-Kent's writing only one species was. 

 known ; we now know about seventy. [C.D. F.] 



