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 position, 
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 Euscorpius. 
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 suggested 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). 
