MUSEUM OF COMPARATIVE ZOOLOGY. 53 



the hypodermal nuclei those of the cone-cells, thus giving four instead 

 of two. 



Tlie four nuclei in the corneal hypodermis of each omrnatidium, as 

 described by Nusbaum in the case of Mysis, had already been seen by 

 Greuacher, who further stated that the nuclei were aiTanged in pairs at 

 two levels, Grenacher does not describe nuclei in the two segments of 

 the crystalline cone. In the retina of Mt/sis stenolejns, the four nuclei 

 which were described by Grenacher are easily identified, but I cannot 

 agree with Grenacher's statement ('79, p. 118) that all four belong to 

 the same category. The more superficial pair unquestionably belong to 

 the corneal hypodermis; but the deeper pair, I feel confident, are the 

 nuclei of the crystalline cone. The conclusion to be drawn from this 

 interpretation of the work of Eeichenbach, K^usbaum, and Grenacher is, 

 that in Decapods and Schizopods each omrnatidium possesses two cells 

 in the corneal hypodermis and only two. 



Concerning the number of cone-cells in the ommatidium of Decapods, 

 different writers very generally agree. Eeichenbach, Kingsley, Herrick, 

 and Patten all state that there are four cone-cells in the Crustaceans 

 which they have studied. 



In the cone-cells the number four, according to Grenacher, is charac- 

 teristic not only of Decapods, but, excepting the Schizopods, it is the 

 distinguishing feature of the Podophtlialmata. In Mysis, Grenacher 

 states that the cone has two, instead of four segments. I have studied 

 the eyes of Mi/sis stenolepis, Smith, and my observations confirm this 

 statement. Notwithstanding Grenacher's assertion, Xusbaum ('87, 

 p. 1 79) claims that the nuclei of the cone-cells in Mysis are grouped 

 ill fours. I am confident that there are only two nuclei in the adult 

 cone, and having seen no evidence of a suppression of two nuclei, I 

 must consequently side with Grenacher in his belief that the cone of 

 Mysis is composed of only two cells. 



The differentiation of the retinula; into distal and proximal groups is 

 more complete in the lobster than in the majority of other Decapods 

 studied. As a result of this incomplete differentiation, it is often 

 impossible to get exact statements from the descriptions of different 

 authors concerning the number and position of the retinulte. 



Eeichenbach ('8G, p. 93) maintains that in Astacus, after the four 

 cone-cells, and as he believes the four hypodermal cells, were differ- 

 entiated, the remaining cells became pigment-cells (retinula;). Essen- 

 tially the same account is given by Nusbaum ('87, p. 180) for Mysis. 

 Kingsley ('87, p. 53) states that, in addition to the corneal hypodermis, 



