MUSEUM OF COMPARATIVE ZOOLOGY. 101 



The retinular cells in Mysis are of two kinds, proximal and distal. 

 The proximal cells extend from the basement membrane distally to 

 the level at which the cone rapidly contracts. The pigment which they 

 contain is for the most part concentrated around the rhabdome, and 

 their nuclei occupy a distal position in the cell (Fig. 73, nl.px.). 



In Mysis the number of cells comprising the retinula is at least seven 

 (Figs. 85-87). Possibly, as I have elsewhere suggested (Parker, '90 tt , 

 p. 55), the total number of cells in this retinula, as in that of Homarus, 

 may be eight. 



In order to determine this question, I have counted the number of 

 nuclei in several retinuloe of Mysis. The enumeration of these can be 

 easily followed in Figures 79 to 82. These figures represent successive 

 transverse sections through four ommatidia, in the region occupied by 

 the proximal retinular nuclei. The axis of each ommatidium is marked 

 by the fibrous portion of the cone cells (cl. con.), and the same omma- 

 tidium is designated in different sections by the same Roman numeral. 

 The nuclei in ommatidium II. can be counted the most readily. In 

 Figure 79, which represents the most distal section of the series, the 

 cone in ommatidium II. is surrounded by a circle of six nuclei, which 

 have been numbered from 1 to 6. Each of these nuclei, however, par- 

 ticipates in three circles (compare nucleus 5), and hence only two of the 

 six can be referred to ommatidium II. Two similar circles occur, one 

 in the sections shown in Figure 80, and one in that shown in Figure 81. 

 As in the former instance, two nuclei in each circle belong to omma- 

 tidium II. In these three circles, then, there are in all six nuclei to be 

 allotted to ommatidium II. In addition to these nuclei, it will be no- 

 ticed that to the right of the cone in Figure 80 there is one more 

 nucleus (No. 7), and still another in a similar position in Figure 82. 

 These two nuclei, when added to the six already summed up for om- 

 matidium II., make a total of eight nuclei for this ommatidium. 



The same number of nuclei occurs in each of the other three omma- 

 tidia, but their arrangement is not quite so regular as in the one just 

 counted. From this I conclude that the number of nuclei in a retinula 

 of Mysis is eight. 



The different nuclei in this retinula usually present a very uniform 

 appearance. The most proximal one differs somewhat from the others 

 in being more elongated (compare Figs. 73 and 82). The seven distal 

 nuclei, on account of their general resemblance, belong, I believe, to the 

 seven functional retinular cells. The single proximal nucleus probably 

 represents an eighth rudimentary cell. The position of this nucleus, 



