98 BULLETIN OF THE 



In accordance with the facts already presented, the number of cells 

 contained in the ommatidium of Serolis can be stated as follows : cells 

 of the corneal hypodermis, two, with possibly two others interomma- 

 tidial in position ; cone cells, two ; retinular cells, six, two distal and 

 four proximal ; hyaline cells, one or two ; a variable number of small 

 pigment cells of ectodennic (?) origin. 



Leptostraca. 



The histological structure of the ommatidia in the Nebaliie has been 

 investigated, so far as I am aware, only by Claus ('88, pp. G5-84). I 

 have had no material for the study of the eyes in these Crustaceans, 

 and I can therefore only present, in the form of a summary, the more 

 important results of Claus's exhaustive study. 



In Nebalia there is a corneal hypodermis (Clans, '88, pp. 68 and 69), 

 the cells of which are grouped in pairs. As in many of the higher 

 Crustaceans, there is one pair of these cells for each ommatidium. The 

 corneal cuticula is facetted ; the outlines of the facets are circular, and ad- 

 joining facets are separated from one another by a small amount of inter- 

 vening cuticula (Claus, '88, Taf. X. Fig. 10). The cones are composed 

 of four segments (Claus, '88, p. 69). The structure of the retinula is 

 somewhat complex. The greate'r part of the rhabdome is surrounded 

 by seven retinular cells. Distal to these cells, however, are seven pig- 

 ment cells, which enclose the proximal prolongation of the cone cells and 

 the distal end of the rhabdome. Such a relation between pigment cells 

 and retinular cells is not of common occurrence among Crustaceans, and 

 it is possible that the bodies which Claus has taken for pigment cells are 

 really the distal ends of the retinular cells. Claus describes and figures 

 what he believes to be the nuclei of both kinds of cells, but I think 

 his figures fail to show that these nuclei are within the limits of the 

 cells to which they are said to belong. It seems to me quite possible 

 that what he has described as two circles of seven cells each may 

 be merely one circle seen at two different levels, as the correspondence 

 in numbers suggests. This single circle would be of course composed 

 of retinular cells, the nuclei of which are probably the distal ones of 

 the Iwo sets described by Claus. The proximal nuclei, which, accord- 

 ing to Claus, belong to the retinular cells, occupy positions not unfre- 

 quently taken by the nuclei of accessory pigment cells, and I am inclined 

 to think that such is their real nature. This interpretation would be 

 more in accordance with the conditions found in ommatidia which have 

 seven retinular cells than is the one given by Claus ; but as I have not 



