MUSEUM OF COMPARATIVE ZOOLOGY. 55 



that there are seven retinulse in the ommatidia of this Crustacean coin- 

 cides fairly with the results obtained from the lobster. 



From the histological evidence of the adult, on the other hand, writers 

 are generally agreed that the ommatidia of Decapods possess seven 

 proximal retinulee. It is probable, however, that this statement requires 

 some qualitication. It will be recalled that, in describing the proximal 

 retinulte of the lobster, I referred to an additional nucleus which ap- 

 parently represented an eighth rudimentary retinula. I have identified 

 this eighth nucleus in the ommatidia of Cambarus, where, as in the 

 lobster, it lies in a plane different from that of the other seven nuclei. 

 If the eighth nucleus should be present in approximately the same 

 plane as the other nuclei, it could be identified only with great difficulty. 

 It is my belief that it often occurs in this position, and probably 

 for this reason it has generally escaped the attention of investigators, 

 for I am of opinion that it is present in the ommatidia of all Decapods. 

 When, therefore, the statement is made that the ommatidia of a certain 

 Decapod contains seven proximal retinulce, the probabilities are that 

 the ommatidium in reality contains eight proximal retinulae, one of 

 which is rudimentary. 



Concerning the Schizopods, Grenacher states ('79, p. 119) that in 

 Mysis there are more than four .proximal retinula?, but how many more 

 he is not certain. In Mi/sis stenoiepis my own observations have shown 

 me that there are certainly seven pigmented proximal retinulae. This 

 number agrees with tlie number of functional retinulas in Decapods, and 

 my opinion is that in Mysis, as in Decapods, an eighth rudimentary 

 proximal retinula may be expected. 



The presence of distal retinuhe in the ommatidia of Decapods seems 

 to have generally escaped attention. Patten and Carriere, however, 

 describe these cells in Penaeus and Astacus, and the fact that they are 

 easily recognized in the eyes of Homarus, Cambarus, and Eupagurus 

 inclines me to the belief that they form a constant element in the om- 

 matidia of all Decapods. They have also been seen in the retina of 

 Mysis. In the eyes of this genus their nuclei occupy the position indi- 

 tated by d in Grenacher's Figure .12. It is of interest to observe that 

 their permanent position in Mysis is an early and transitory one in the 

 lobster. (Compare Grenacher, '79, Fig. 112 J, with Figs. 48 and 55 of 

 this paper.) In both the Decapods and Mysis the number of distal 

 rctinulai is two. 



