ro8 THE APODIDvE PART I 



crowding between the retinae. Fig. 28 is a diagram to 

 show the way in which we suppose the unpaired eye 

 to have been formed out of a pair of simple hypo- 

 dermal eyes. Further investigation must decide 

 whether the dorsal and ventral retinae and the larger 

 posterior cell groups of the lateral retinae are later 

 differentiations of the same two original eyes or new 

 developments. In the inner ends of the retinal cells, 

 i.e., in that part of the cells which points inwards, 

 irregular roundish or oval refractive bodies of different 

 sizes are found ; these are probably remains of rods or 

 rhabdomeres originally secreted by these retinal cells. 



The whole structure of the organ, its apparently 

 loose connection with the hypodcrmis, the chitinous 

 fold which runs down as a branch from the water-canal 

 into its cavity, the occasional occurrence of brown 

 eye pigment instead of the olive green connective 

 tissue pigment, the nerves of its retinae running 

 separately into the brain, all tend to support the 

 above view of its origin (cf. also p. 169). 



Further corroboration of this theory of the origin of 

 the unpaired eye from the anterior pair of Annelidan 

 eyes will be found in the section on Limulus. In that 

 animal the anterior Annelidan eyes remain as eyes, but 

 are reduced to ocelli, or eyes with one single large cuti- 

 cular lens. These ocelli first appear, according to 

 Packard, on tJie ventral surface^ and wander on to the 

 dorsal surface in the course of the later development. * 

 This astonishing fact receives its full explanation if we 



1 We shall also find clear traces of a migration of the eyes in the 

 Crustacean Nauplius, xi. Figs. 36 and 37, p. 158. 



