THE EYES OF HYDRACHNIDA 



129 



lateral eyes, is developed from the covering hypoderm or ectoderm cells, 

 and a thickening of the cuticle gives rise to the superficial corneal lens. 

 The early stage of the development of the median eyes is thus one which 

 corresponds to the development of an inverted retina such as is found 

 among the Mollusca, in the Scallop or Pecten ; and since the median eyes 

 of adult spiders are as a rule of the simple upright type, it would be 

 interesting to obtain further information with regard to the later stages of 

 development and the structure of the adult eyes of Agelena. 



The Eyes of Hydrachnida 



A description of the eyes of freshwater mites was published by P. Lang 

 in 1905, which affords a most instructive demonstration of widely 

 divergent developments of the eyes of closely allied species : more 

 especially with regard to transitions between eyes which are fixed in 



Diplodontus Elyais Hydrodroma 



Fig. 91. — Diagrams showing the Position and Direction of the Ocelli, or 

 Simple Eyes, of Different Species of Hydrachnida (Fresh- water Mites). 

 In the genus Hydrodroma a " Fifth " Median Eye is present on the 

 Dorsal Aspect of the Head Shield. (After P. Lang.) 



definite positions — the cuticular lens being continuous with or connected 

 to the adjacent cuticle — and eyes which are enclosed within a chitinous 

 covering, within which they are capable of movements produced by 

 muscles arising from the wall of the cavity and inserted into the eyeball ; 

 a condition which is somewhat similar to that of the lateral eyes of verte- 

 brates, which being enclosed and protected by the walls of an orbital 

 cavity are capable of movements within the capsule of Tenon, which are 

 produced by special ocular muscles. 



The eyes vary in number in different species of the Hydrachnida, 

 which species were originally classed by Muller in three subdivisions : 

 (a) oculis binis ; (b) oculis quator ; (c) oculis sex. Lang describes the 

 eyes of Limnesia undulata, Curvipes carneus, and Hygrobates longipalpis 

 as lying completely inside the cuticle. In Limnesia (Fig. 91) the eyes of 

 the two sides are widely separated — the optic axes of the anterior pair are 



