MUSEUM OF COMPARATIVE ZOOLOGY. Ill 



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 Palaemon, Grenacher ('77, p. 32) 

 has demonsti-ated that the rhabdome is surrounded by seven retinular 

 cells. He also ('77, p. 33, and '79, p. 125) observed the same number 

 in the retinulse of Astacus and Portunus. Since the publication of 

 Grenacher's observations, a retinula containing seven cells has been 

 seen in Astacus by Carriere ('85, p. 169), in Penseus, Palsmon, 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 Palasmonetes, Palinurus, 

 Cambarus, Crangon, and Cancer, I have succeeded in ascertaining the 

 number both of the functional cells and of the nuclei in the retinulse. 



In Palaeraonetes each rhabdome is surrounded by at least seven re- 

 tinular cells (Plate TX. Fig. 114, cl. px.). The nuclei of these cells 

 usually lie slightly distal to the rhabdome (Fig. 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 Palsemonetes, 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. 



