240 Prof. Dr. Fr. Vejdovsky on 



lias been described by L. v. Graff as a distinct species — 

 Gyrator coccus. 



Tlie foregoing cases may support the opinion expressed by 

 Packard, and more recently by Chilton, that it is possible 

 that different specief^, and even individuals from different 

 localities, may exhibit different stages in the reduction of the 

 eyes. 



The discovery of Bathyonyx offers now excellent evidence 

 as to the probable fiist stage in the degeneration of the eyes. 

 The ancestors of this genus were certainly closely related 

 to a Gammarus with normal eyes, as may be seen from 

 the \vliole organization and in particular from the capsular 

 segmental cuticular sense-organs. Tlie visual organs also 

 represent the well-known kidney-shaped eyes of a Gammarus- 

 like ancestor. The descendants, adapting themselves to live 

 in the dark depths of the lake *, gradually lost the power 

 of seeing with the compound eye, as individual ommatidia 

 remained functionless and consequently the crystalline cones 

 became subject to degeneration. Only the pigment-cells 

 remained and the ommatidia which still retained their 

 functional power appeared then as scattered components of an 

 eye on each side of the head. It is in this stage that 

 Bathyonyx now presents itself to us. 



But the crystalline cones still present liave also undergone 

 degeneration, as is shown by the little homogeneous spheres 

 within the cones. The hypothesis of the progressive re- 

 duction of the eyes can, 1 believe, be supported by the 

 following facts : — 



(1) From the variable number of the crystalline cones in 

 one and the same animal on the right and left sides of the 

 head. 



(2) From the extremely variable size of the individual 

 crystalline cones, which in one and the same eye present 

 instances of diminution in size so as to finally appear simply 

 as little refractive particles. 



(3) From the form of the crystalline cones themselves. 

 The homogeneous spheres within the fine-grained matrix 

 must, I think, be regarded as evidence of degeneration. In 

 this way the crystalline cones lose tlieir dioptric property. 

 The degeneration, however, does not occur in any particular 

 order, but may affect widely separated ommatidia, while the 

 intermediate cones may remain intact. 



In consequence of the foregoing, the originally compact 



* From the small number of specimens obtained it may be supposed 

 that the species lives perhaps in the mud and only occasionally swims 

 nboiit freelv in the water. 



