102 BULLETIN OF THE 
proximal to the other retinular nuclei, is similar to that occupied by the 
nucleus of the rudimeutary retinular cell in Homarus (compare Parker, 
90°, pp. 20, 21). 
The rhabdome in Mysis stenolepis lies in the proximal portion of the 
retina. It is rather stout,,blunt at its distal end, but sharper proxi- 
mally (Fig. 90). Its surface is marked with coarse corrugations. In 
transverse section, its outline is a square; this is subdivided by two 
lines into four smaller squares, a condition already observed by Grena- 
cher (’79, p. 119) in M. flexuosa, The relation of the retinular cells 
to these divisions of the rhabdome can be clearly seen in Figure 87. 
According to Grenacher’s account (’79, p. 118), a rod-like structure 
extends, in Mysis vulgaris and M. flexuosa, through the axis of the 
ommatidium from the distal end of the rhabdome to the region of the 
proximal retinular nuclei. Whether this rod be a proximal continuation 
of the cone, or a distal extension of the rhabdome, Grenacher found it 
difficult to decide. He is inclined, however, to the former opinion, 
A similar structure occurs in the ommatidia of Mysis © stenolepis. 
Although I have made repeated attempts, I have never succeeded in 
isolating the rod in connection with either the rhabdome or the cone 
cells. In transverse sections, the distal end of it appears in a position 
slightly proximal to the retinular nuclei (Figs. 73 and 83). The cone 
cells extend proximally as a transparent axis to this region, and the 
most distal indications of the rod are four fibres which lie on the 
periphery of what I take to be the proximal end of the cone cells 
(Fig. 83). Somewhat deeper than this, the four fibres thicken, and 
finally fuse (Fig. 84), producing a body which in transverse section has 
-the outline of a four-pointed star. In a plane slightly more proximal, 
the outline changes to a squarish one (Fig. 85), and this is retained 
almost to the proximal end of the rod. Throughout its extent, this 
problematic rod is closely surrounded by the seven proximal retinular 
cells (Fig. 85). It is separated from the rhabdome by what appears to 
be an open space (Fig. 90, at the level of the dotted line 86). In trans- 
verse sections (Fig. 86), however, this space is seen to be divided by 
delicate membranes into four compartments. 
These facts, however, do not aid much in deciding the relationship 
of the rod. The fact that it shows indications of being composed of 
four parts suggests its connection with the rhabdome. The four parts 
of which it consists do not, however, correspond in position to the seg- 
ments of the rhabdome, but fall between them. (Compare Figs. 83 and 
87.) On the other hand, if it were an extension of the cone, one would 
