120 BULLETIN OF THE 



meat membrane, and probably passes through it in company with the 

 fibrous ends of the retinular cells (compare Parker, '90', pp. 17-19). 

 Admitting that these cells are merely modified accessory pigment cells, 

 such a condition as this is quite unintelligible to me ; but granting 

 them to be differentiated retinular cells, their fibrous extensions can 

 be easily explained as the rudiments of the fibrous portion of the cell 

 with which the nerve fibre was once connected. A somewhat similar 

 case occurs in Mysis, where the centre of each of the pigment cells 

 which surround the cone contains a small transparent axis. This axis 

 in every respect except that of connection with a nerve fibre corresponds 

 to the fibrillar axes described in the functional retinular cells of this 

 Crustacean (compare Plate VII. Figs. 77, 78, and 87). Consequentlj', 

 the axis in the distal cells either represents a rudimentary nervoas axis, 

 in which case the cell containing it must be regarded as a retinular cell, 

 or it is something for which I can suggest no explanation. 



These facts lead me to conclude that the pigment cells which sur- 

 round the cone in Scrolls, the Stomatopods, Schizopods, and Decapods, 

 are to be regarded as modified retinular cells, and I have therefore 

 described them under the name of distal retinular cells, in contrast to 

 proximal retinular cells, or those which retain their primitive position 

 around the rhabdome. In the differentiation of a group of simple 

 retinular cells into proximal and distal cells, the latter necessarily 

 change their function from that of terminal nervous organs to that of 

 screens chief!}' concerned in excluding the light from the sides of the 

 cones. Wherever the distal retinular cells occur, they afford evidence, 

 I believe, that the structure of the ommatidium has undergone a modi- 

 fication fi'om the primitive ommatidial condition. 



The second method by which the structure of omraatidia may be 

 changed, namely, the suppression of cells, is perhaps the one whose 

 presence is most easily detected because of the frequent persistence of 

 the partially reduced cells. These rudimentary cells can be identified 

 most readily in the cases where they belong to groups in which the 

 number of elements is constant for different ommatidia. I know of no 

 evidence of suppression among the groups of cells in the corneal hypo- 

 dermis or the cones. Among the retinulte, however, it seems to be of 

 rather common occurrence. The first indication of this process is natu- 

 rally a diminution in the size of the cell to be suppressed. Such a step 

 is perhaps shown in the retinula of Gammarus (Plate I. Fig. G), where 

 one of the five cells, although evidently functional, is nevertheless con- 

 siderably reduced. Without much doubt, the body described in the 



