MUSEUM OF COMPARATIVE ZOOLOGY. 111 
in this genus, as in Herbstia, it was maintained that there were only 
four cells. Subsequent investigators have not confirmed this conclusion. 
In transverse sections of the retinula of Palemon, Grenacher (’77, p. 32) 
has demonstrated that the rhabdome is surrounded by seven retinular 
cells. He also (’77, p. 33, and ’79, p. 125) observed the same number 
in the retinule of Astacus and Portunus. Since the publication of 
Grenacher’s observations, a retinula containing seven cells has been 
seen in Astacus by Carriére (’85, p. 169), in Penzus, Palemon, Gala- 
thea, and Pagurus by Patten (86, pp. 630 and 643), and in Cambarus 
by Watase (’90, p. 299). 
In Homarus, as I (’90*, p. 21) have already shown, the retinula con- 
tains, in addition to the seven functional retinular cells, an eighth rudi- 
mentary one, which is little more than a nucleus. In order to ascertain 
the presence or absence of this eighth cell in other Decapods, I have 
been careful to record the number of retinular nuclei, as well as the 
number of functional retinular cells. In some genera, such as Cardisoma 
and Hippa, I have not been able, on account of the unfavorable condition 
of the tissue, to make this determination ; but in Palemonetes, Palinurus, 
Cambarus, Crangon, and Cancer, I have succeeded in ascertaining the 
number both of the functional cells and of the nuclei in the retinule. 
In Palzemonetes each rhabdome is surrounded by at least seven re- 
tinular cells (Plate IX. Fig. 114, el. px.). The nuclei of these cells 
usually lie slightly distal to the rhabdome (Tig. 104, nl. px.). Their 
arrangement is shown in Figures 110, 111, and 112, which represent a 
series of consecutive sections through the region occupied by the prox- 
imal retinular nuclei of five ommatidia. The nuclei of the different 
ommatidia are arranged upon the same plan, and the corresponding 
nuclei in the different sets have been marked by the same number. In 
several instances, nuclei have been cut in two, and their parts are found 
in consecutive sections ; in such cases the separate portions have been 
marked with the same number. As can be seen in these figures, 
the number of nuclei in the distal portion of each retinula is seven. 
But in addition to these, there is also another one, which occupies a 
position near the rhabdome. This nucleus resembles the others in all 
respects except that it is somewhat longer and narrower. It is drawn 
in Figure 103 at the level marked 114, and in Figure 114 one can see 
the regularity with which it occurs. This nucleus is the eighth in the 
retinula of Palemonetes, and since it differs somewhat in structure from 
the other seven, and occupies a more proximal position, I believe it rep- 
resents a rudimentary retinular cell. 
