102 BULLETIN OF THE 



proximal to the other retinular nuclei, is similar to that occupied by the 

 nucleus of the rudimentary retinular cell in Homarus (compare Parker, 

 '90 a , pp. 20,21). 



The rhabdome in Mysis stenolepis lies ir 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- 

 dier ('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 Grenadier'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, Grenadier 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 8G). 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 



