MUSEUM OF comparatrt: zoology. 95 



vanced embrj'os (compare nl. dst., in Figs. 65 and G9). In transverse 

 sections at tiiis stage (Fig. 69) each cone is surrounded by a circle of 

 six nuclei. Each nucleus, however, participates in three adjoining cir- 

 cles, consequently there are only twice as many nuclei as ommatidia. 

 In the adult the nuclei of these pigment cells (Fig. 60, nl. dst.) occupy 

 the same relative positions as in the embryo; in the latter, however, they 

 are usually somewhat hidden by tlie pigment which surrounds them. 



In the embryo the nuclei of the pigment cells surrounding the cone 

 resemble very closely, except in point of size, the nuclei of the retinular 

 cells (compare nl. dst. and nl. px. in Fig. 65). In the nuclei of the 

 retinular cells there is usually one distinct nucleolus, sometimes two, but 

 as a rule no finer particles. This condition also obtains in the nuclei of 

 the pigment cells. Not only are the nuclei of these two kinds of cells 

 similar in the embiyo, but they are also much alike in the adult (com- 

 pare nl. dst. in Fig. 60 with nl. rtnJ in Fig. 63). 



Because of this resemblance, I believe that the pigment cells which 

 surround the cone can be fairly considered to be modified retinular cells, 

 which have lost their sensory function in precisely the same way as in the 

 case of the distal retinular cells in Decapods (see Parker, '90*, p. 57). If 

 this interpretation of the pigment cells be accepted, it follows that in 

 Serolis, as in Decapods, two kinds of retinular cells are present, proximal 

 and distal, and that the primitive ommatidium from which that of Serolis 

 was derived probably contained six retinular cells functional as nervous 

 structures. It need scarcely be added, that this number is characteristic 

 for the ommatidia of many Isopods. 



The retiuula in the species of Spheeroma which I studied presents 

 an appearance which suggests the differentiation of simple retinular cells 

 into proximal and distal cells. In Sphferoma there are seven retinular 

 cells (Plate V. Fig. 58) ; three of these are considerably reduced ; the 

 remaining four are large, and recall the four retinular cells of Serolis. 

 In transverse sections it can be shown that the four large cells in Sphre- 

 roma not only resemble in appearance the four proximal cells in Serolis, 

 but that they occupy the same relative positions in the ommatidium. 

 In Serolis the plane which separates the two cone cells of any given 

 cone, when extended, separates the four proximal retinular cells into two 

 groups of two cells each (compare Plate YI. Fig. 68 with Figs. 71 and 

 72). The plane of separation in the cone of Sphseroma divides the retin- 

 ula by passing through the single small retinular cell shown in the lower 

 part of Figure 58 (Plate V.) and hetiveen the two small cells on the oppo- 

 site side, thus separating the four large retinular cells into two groups, 

 as in Serolis. 



