116 BULLETIN OF THE 



under the head of Cone Cells indicates the occasional occurrence of cones 

 containing only four cells, although the usual number is five. In the 

 line fur Serolis, under the head of Corneal Hypodermis, the parenthesis 

 and included signs are intended to indicate the possibility of there being 

 more than two cells in the corneal hypodermis for each ommatidium. 

 In the Schizopods, Stomatopods, and Decapods, the number of prox- 

 imal retinular cells is expressed in the form of 7 + 1 instead of 8, be- 

 cause one of the cells is rudimentary. 



The Innervation of the Retina. 



The innervation of the retina in the compound eyes of Crustaceans is 

 chiefly interesting, because of its importance in relation to physiological 

 questions. As this paper deals with a morphological topic, it would be 

 obviously irrelevant to enter upon any extended discussion of this sub- 

 ject. Nevertheless, the innervation of the retina is not without some 

 bearing on the general question which I have set for myself, and I shall 

 therefore not pass it by, but put in as brief a form as possible what I have 

 observed concerning it. 



In my account of the retina in the lobster, I described the optic- 

 nerve fibres as terminating in the proximal retinular cells. Near the 

 ganglion each fibre consists of a bundle of fibrils, simply enclosed 

 within a sheath, but as it approaches the retina it becomes coated with 

 pigment. The pigment increases in quantity and the fibre correspond- 

 ingly enlarges till it finally becomes continuous with the deeply pig- 

 mented retinular cell. The fibrillar axis can be distinguished in the 

 pigmented portion of the fibre as a transparent axial structure, and it 

 can also be traced distally through the pigment of each retinular cell 

 till it breaks up into its ultimate fibrillar, which are spread over the dis- 

 tal half of the rhabdome. This is the method of nerve termination in 

 the lobster, and points very conclusively to the rhabdome as the termi- 

 nal organ. 



What I have seen of the termination of the nerve fibres in other 

 Crustaceans confirms the account which I have already given for the 

 lobster. In some species which I have studied, owing to the small size 

 of the retinal elements, 1 was unable to determine the cells with which 

 the nerve fibres connected. The termination of the fibres in the cells 

 of the retinula was observed, however, in the following genera : Bran- 

 chipus, Limnadia, Pontella, Gammarus, Talorchestia, Idotea, Porcellio, 

 Spheeroma, Serolis, Gonodactylus, Mysis, Palaemonetes, Crangon, Cam- 



