264 W. SAVILLE-KENT, CONTRIBUTIONS TO A 



The question has naturally arisen from this interpretation as ta 

 whaD has become of the missing pair of antennae, and although 

 at a first glance no satisfactory answer to this inquiry would 

 appear to be forthcoming from the higher Arachnida, some 

 evidence of importance is apparently to be derived from a study 

 of the small group of the Hydrachnida. In a considerable 

 number of these, as for example the genera Limnesia, Hydrachna^ 

 Eulais and Diplodontus, two more or less remotely separated 

 pairs of simple eyes are developed ; while in a second series, 

 including such genera as JVesaea* Fiona, Marica ?i\'\(\. Ai^rhenurus,. 

 but a single pair of visual organs are distinctly recognisable. 

 According to the recent observations of C. J. Neuman t and 

 H. Lebert, i two pairs of eyes represent the normal number 

 possessed by all the members of the group, but the two 

 are so close together that the pigment masses coalesce with 

 one another and thus present superficially the aspect of a single- 

 pair only, their compound character, however, being rendered 

 evident by the presence of a double cornea. Without being in 

 a position precisely to endorse this interpretation the presence of 

 a second corneal element being indeed undemonstrable in many 

 species examined I have succeeded in tracing the connection 

 of the two eyes on either side, as developed in Limnesia 

 histrionica with a single nerve-cord that simply branches to- 

 wards its distal end, giving off a twig to each ocellus. This 

 fact tends to show that even when developed distinctly in 

 duplicate these visual elements must be regarded as tha 

 homologues only of a simple optic organ, or may indeed be 

 appropriately correlated with an elementary condition of the- 

 compound eyes of many insects and Crustacea, assuming each 

 eye-like structure in the Water-mite to be the equivalent of 



* For the generic and specific names as now understood we must sub- 

 stitute the following", an alteration which will answer all through this paper : 

 For Nesaea use Puma ; for Piona use Acercus ; for Marica use Frontipoda ; 

 for Atax use TJnionicula ; for Pseiuloatax use Neumanla ; for Brachjhates 

 use TJujas. For Limnesia histrionica use L. fidtjida ; for Diplodontiis 

 Jilipes use I). de.y}iciens ; for Mideoj)sis de2yressa use M. arbicularis. 

 Anurania. as a generic name is not now used, as it was only a nymph stage 

 in an Arrhenurus. [CD. 8.] 



f " Sveriges Hydraclinider," Kongl. Sv. Vet. Akad. Handl. Bd. 17,. 

 1880. 



X " Hydrachnides du Lac Leman," Bull. Soc. Vaud., t. xvi., 1879. 



