66 BULLETIN OF THE 



in connection with this crowding of the cones, a condition found for the 

 most part only in macrurous Decapods. 



In accounting for the rearrangement of the ommatidia, the eyes in 

 the Stomatopod Gonodactylus afford some important evidence. As I 

 have previously mentioned, the ommatidia in this genus are of two sizes. 

 The larger ones have several of the peculiarities characterizing the tetrag- 

 onal arrangement : their facets are generally square ; they are arranged 

 in single lines, and these lines, so far as the relations of the individual 

 ommatidia are concerned, show evidences of having slipped upon one 

 another. The smaller ommatidia have hexagonal facets, and are clearly 

 arranged according to the hexagonal plan. The larger ommatidia are 

 rather closely packed; the smaller ones are arranged with more open 

 space between them (compare Plate VIII. Fig. 93). In this genus, 

 then, as in the lobster, the tetragonal arrangement occurs in connec- 

 tion with the crowding of the ommatidia. 



How an increase in size, accompanied by a crowding of the retinal 

 elements, can induce the change in arrangement which seems to follow 

 it, I am at a loss to explain. Nevertheless, the two phenomena ap- 

 pear to be in some way connected. 



From the pi'eceding discussion concerning the arrangement of the 

 ommatidia, the following conclusions can be drawn. The ommatidia, 

 when numerous enough, present one of two plans of arrangement, 

 the hexagonal or the tetragonal. The hexagonal plan is phylogeneti- 

 cally the older, and is characteristic of the eyes of all Crustaceans 

 except some families of the macrurous Decapods, especially the Gala- 

 theidse, Palinuridse, Astacidse, and Carididse. In these the hexagonal 

 arrangement is usually replaced by the tetragonal; but in the adults of 

 some species, especially those in which the eyes are partially rudi- 

 mentary, the hexagonal arrangement persists. The change from the 

 hexagonal to the tetragonal arrangement is connected apparently with 

 an increase in size, and consequent crowding, of the ommatidia. 



The Structure of the Ommatidia. 



Each ommatidium, as I have previously mentioned, consists of a 

 cluster of cells more or less regularly arranged about a central axis. 

 The crreatest number of kinds of cells which an ommatidium is known 

 to contain is five. These are the cells of the corneal hypodermis, the 

 cone cells, the proximal and distal retinular cells, and the accessory 

 cells. 



