6 BULLETIN OF THE 



In the ommateum the cells are arranged in specialized groups or 

 ommatidia. There is a single ommatidium under each corneal facet, 

 consequently in any given eye the number of ommatidia equals the 

 number of facets. The cellular composition of each ommatidium is 

 best understood from a comparison of longitudinal and transverse sec- 

 tions. Figure 1 represents a longitudinal section through an ommatid- 

 ium. The thick lamellated layer (cm.) at the distal end is the corneal 

 cuticula. Directly below this is a thin layer of cells, the corneal hypo- 

 dermis (cm. hd.). Following on the corneal hypodermis are the cone- 

 cells (cl. con.). They are very long, and extend from the corneal hypodermis 

 inward till their proximal ends disappear in the deep part of the retina. 

 In reality they terminate upon the basement membrane. Their distal 

 ends in the region of the crystalline cones are surrounded by pigment- 

 cells, to which I give the name distal retinulse (rtn r . dst.). These, like 

 the cone-cells, extend to the deeper part of the retina. Here the proxi- 

 mal retinulse and accessory pigment-cells occur. The proximal retinulse 

 are elongated cells (rtn 1 . px.), and contain black pigment. They sur- 

 round the rhabdomes (rhb.). The accessor}' pigment-cells are irregular 

 cells, which fill the space between the deep ends of the proximal reti- 

 nulse. They contain a pigment which is whitish by reflected and yellow- 

 ish by transmitted light. Their nuclei are shown at nl. pig.^ Figure 1. 

 The last two kinds of pigment-cells described rest upon the basement 

 membrane (mb.); below this membrane the fibres of the optic nerve 

 can be seen (n.fbr.). 



From this description it will be seen that the ommateum lies between 

 the corneal cuticula and the basement membrane, and is composed of 

 the following kinds of elements : cells of the corneal hypodermis, cone- 

 cells, distal retinulse, proximal retinulae, and accessory pigment-cells. 

 The numbers and positions of these cells are best made out from trans- 

 verse sections. The several kinds of cells will be discussed in the order 

 named. 



The Corneal Hypodermis. 



That the corneal cuticula in Decapods is separated from the cone-cells 

 by an intervening layer of cells is a view which has been held only by 

 recent investigators. Grenacher ('79, p. 123), in his account of the eyes 

 in Decapods makes no mention of such a layer, and leaves one to con- 

 clude that the cone-cells abut against the cuticula. Claus ('86, p. 57) 

 suspected the presence of a corneal hypodermis in Decapods, Schizopods, 

 andStomatopods, but his search for it was in vain. 



