202 BULLETIN OF THE 



cells from the deep ends of which the nerve fibres emerge. Admitting 

 that in the ancestral eye the rhabdomeres were in their usual positioni 

 namely, at the outer end of each retinal cell, an inversion of this retina, 

 would not only place the optic fibres on the front face of the retina, but 

 the rhabdomeres would come to occupy the deep ends of the cells. The 

 prenuclear rhabdomeres of a normal retina would, therefore, be homolo- 

 gous with the postnuclear rhabdomeres of an inverted retina. The 

 prenuclear rhabdomeres of the median eyes in scorpions must, then, be 

 secondary structures, developed in such a way as to replace functionally 

 the older postnuclear structures.* 



The phaospheres, as Mark ('87, p. 93) has already suggested, may rep- 

 resent the remains of postnuclear rhabdomeres. These are to be regarded, 

 then, in the nature of disappearing organs, and the fact that in some 

 species of scorpions they are present, while in others they are absent, 

 would favor this view. As Mark has stated, the phaospheres, if they 

 represent postnuclear rhabdomeres, should be found only in eyes with 

 inverted retinas. Lankester and Bourne, as previously mentioned, have 

 described them in the lateral eyes of Euscorpius. Mark hesitated, in the 

 case of the lateral eyes, as to whether he should follow Graber's observa- 

 tions and consider them triplostichous, with inverted retinas, or whether 

 he should follow Lankester and Bourne and consider them monostichous. 

 In the former case the phaosphere might readily represent postnuclear 

 rhabdomeres ; in the latter, this interpretation would be out of the ques- 

 tion. In Centrurus the structure and development of the lateral eyes show 

 conclusively that they are monostichous, and there seems to be small room 

 to doubt that the same is the case with the lateral eyes in Enscorpius. 

 In these eyes, however, Lankester and Bourne claim the presence of phao- 

 spheres. I have had no material from Euscorpius to examine ; but since 

 in Centrurus the median eyes contain phaospheres, while the lateral eyes 

 are devoid of them, it is a matter of interest to see whether, upon further 

 investigation, the presence of phaospheres in the lateral eyes of Euscor- 

 pius is confirmed, or whether that genus, like Centrurus, has phaospheres 

 in the median eyes only. If they should not be found in the lateral 

 eyes, there would still be reason for considering them the remnants of 

 rhabdomeres ; but if they should be found there, this view would be no 

 longer reasonable. 



The possible relation of the median to the lateral eyes in scorpions 

 has already suggest ,d itself, for in pointing out the probable nature of 



* This relation of the structures of the normal and inverted retina has been fully 

 discussed by Mark ('87, pp. 87-94), 



