108 BULLETIN OF THE 



follows : cells of the corneal hypodermis, two ; cone cells, four ; proxi- 

 mal retiuular cells, eight, one of which is rudimentary ; distal retinular 

 cells, two ; accessory cells (mesodermic ?) present. 



Decapoda. 



I have studied the eyes of the following species of Decapods : Gelasi- 

 mus pugilator, Latr. ; Cardisoraa Guanhumi, Latr. ; Cancer irroratus, 

 Say ; Hippa talpoida. Say ; Palinurus Argus, Latr. ; Pagurus longicarpus. 

 Say ; Homarus americanus, Edw. ; Cambarus Bartonii, Fabr ; Crangon 

 vulgaris, Fabr. ; and Paltemonetes vulgaris, Say. I collected much of 

 this material at the Station of the United States Fish Commission at 

 Wood's Holl, Mass. The specimens of Cambarus were obtained in the 

 vicinity of Philadelphia. I am under obligations to ]\Ir. Herbert M. 

 Eichards for specimens of Palaemonetes collected by him at Newport, 

 R. I. A number of eyes of two Crustaceans, Cardisoma and Palinurus, 

 were kindly obtained for me by Mr. Isaac Holden ; they were collected 

 on the coast of Florida by Mr. Ealph Munroe, to whom I am indebted 

 for the careful way in which they were preserved. 



The corneal hyjwd ermis in Decapods was first recognized by Patten 

 ('86, pp. 626 and 642), who observed it in Penseus, Palsemon, Pagurus, 

 and Galathea. Since Patten's announcement of the presence of this 

 layer in Decapods, it has been identified in a number of other genera : 

 in Crangon by Kingsley ('86, p. 863), in Aljiheus by Herrick ('86, p. 43), 

 in Astaciis by Carriere ('89, p. 225), in Cambarus and Callinectes by 

 Watase ('90, pp. 297 and 299), and in Homarus by myself ('90^ p. 6). 

 More recently I have observed it also in Palaemonetes (Plate IX. Fig. 

 103, cl. cm.), Crangon, Cambarus, Palinurus, Pagurus, Hippa, Cancer, 

 and Cardisoma. 



In almost all Decapods in which the arrangement of the cells in the 

 corneal hypodermis has been observed, these elements have been found 

 to be grouped in pairs, and so distributed that each pair occupies the 

 distal end of an ommatidium (compare Figs. 103 and 106, Plate IX.). 

 This arrangement has been observed, either by others or by myself, in 

 the genera mentioned in the preceding paragraph, except Callinectes, 

 in which the exact arrangement of the cells has not been recorded. 

 Reichenbach's statement ('86, p. 91), that in Astacus there are four 

 hypodermal cells under each facet, is probably erroneous, as Carriere's 

 observations show. 



Although Patten was the first investigator who clearly demonstrated 

 the presence of the corneal hypodermis in Decapods, Grenacher, in 1879, 



