COMPOUyO EYES OF ARTIIROPODS. 293 



insortcd into tlio axial space between the tour rhabdomeres. 

 Eaeb cell lias a peculiar refractive nucleus. It lacks any con- 

 nection with the optic nerve fibre, and is suspended from the 

 bottom of the pit at a considerable distance above the basement 

 inen)brane. The "hyaline cell" is a modified cctodermic cell, 

 and is homologous with the rest of the (•ells which enter into 

 the formation of the ommatidium. The cell, however, does not 

 develop pigment granules in itself, nor establish any connection 

 with the central nervous system as does the retinula cell, nor 

 does it secrete chitin on a part of its surftice as do the rest of the 

 ommatidial cells we have considered already. As to the function 

 of this cell I am not able to say anything definitely. It cannot 

 be sensory, having no connection with the nerve centre. Wlien 

 we see, however, that in some species of Serolis the lower ends of 

 the four rhabdomeres are deeply imbedded in the substance of the 

 "hyaline cell," or, in other words, that the rhabdomeres are 

 firmly held together by the substance of the " hyaline cell " 

 at the point wliere they meet, it becomes probable that the cell 

 in question may serve the purpose of a mechanical support, 

 adding firmness to the whole structure. To this point I will 

 recur later on. 



In addition to the cells which form the essential part of.the 

 ommatidium there exists a number of pigmented cells, {pg. c. 

 Fig. la, PI- XXIX). The pigment cells are greatly developed 

 around the vitrellae and closely invest the dioptric portion of the 

 ommatidium from outside. These pigmented cells also are modi- 

 fied ectoderm cells which lie between the adjacent ommatidial 

 pits. 



The morphological unit of the compound eye oi Scrolls may 

 then be described as consisting of a group of ectodermic cells 

 lying outside of the basement membrane, and so arranged as to 

 form the walls of an open tubular depression. This group of cells 

 undergoes special difl\3rentiations at different levels of the pit. 

 The uppermost group, consisting of two flattened cells, consti- 

 tutes the corneagen, which secretes the cornea externally; the 

 second group of cells, consisting of two large cells, the vitrellae, 

 secretes chitin towards the lumen of the tube, thus forming the 

 crystalline cone; the third group of cells, the retinulae, con- 

 sisting of four cells, secretes the rhabdomeres. The retinulae 



