54 BULLETIN OF THE 



each ommatidium in Crangon at first consists of four vertical series of 

 nuclei, each series containing five nuclei. This would give a total of 

 twenty cells for each ommatidium. After deducting four, the number 

 of cone-cells, from twenty, the original number of cells, there remain 

 sixteen cells to be accounted for, presumably as pigment-cells. I have 

 examined the eye of an adult Crangon vulgaris, and I find in it, as in 

 the lobster's eye, two distal retinulse and seven proximal retinulse. This 

 can scarcely be reconciled with Kingsley's account, unless one admits 

 an extensive suppression of cells. Such a suppression seems to me 

 scarcely probable, and I am therefore inclined to believe that there 

 has been some error in Kingsley's method of counting. Possibly 

 the series of nuclei were so placed that they w r ere shared by neigh- 

 boring ommatidia, and did not all belong to one ommatidium. This, 

 however, could only be settled by re-examining the early stages of 

 Crangon. 



Herrick ('86, p. 44) describes seven retinulse in the ommatidium of 

 Alpheus, and makes the statement that they do not possess nuclei. He 

 then describes some undifferentiated ectodermic cells, the nuclei of 

 which can be seen in the space between the cones. As this is the 

 position which in the young lobster is occupied by the nuclei of the 

 proximal retinulse, and as Herrick has not identified any nuclei for 

 the proximal retinulse in Alpheus, I am inclined to regard the deeper 

 nuclei of this group as belonging to these cells. The more superficial of 

 the nuclei described by Herrick are apparently arranged in circles of 

 six around each cone. (Compare Herrick, '86, Figs. 1 and 2.) As in the 

 early stages of the lobster this arrangement was characteristic of the 

 nuclei in the distal retinulse, it is possible that these superficial nuclei 

 in Alpheus may represent the distal retinulse. 



Eeichenbach ('86, p. 92), Kingsley ('87, p. 52), Nusbaum ('87, p. 179), 

 and Herrick ('89, p. 167) describe an ingrowth of mesodermic tissue 

 between the retina and ganglion, and in Mysis, according to Nusbaum 

 ('87, p. 180), these cells, as in the lobster, give rise to what I have 

 called the accessory pigment-cells. 



From the investigations which have been summarized in the preced- 

 ing pages, it is difficult to draw any general conclusion concerning the 

 number of retinulse in the ommatidia of the higher Crustacea. Reichen- 

 bach and Nusbaum make no statement as to the number of these cells 

 in Astacus and Mysis. Kingsley's enumeration of them in Crangon 

 seems to be erroneous. Admitting the undifferentiated ectodermic 

 nuclei in Alpheus to be the nuclei of the retinulse, Herrick's statement 



