120 BULLETIN OF THE 
ment membrane, and probably passes through it in company with the 
fibrous ends of the retinular cells (compare Parker, 790°, 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). Consequently, 
the axis in the distal cells either represents a rudimentary nervous 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 Serolis, 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 chiefly 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 from the primitive ommatidial condition. 
The second method by which the structure of ommatidia 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 retinule, 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. 6), where 
one of the five cells, although evidently functional, is nevertheless con- 
siderably reduced. Without much doubt, the body described in the 
