96 BULLETIN OF THE 



The change which would convert an ommatidium like that in Sphse- 

 roma into one like that in Serolis is easily imagined. It would consist 

 in the complete abortion of one of the three small retinular cells, and the 

 conversion of the other two into the pigment cells surrounding the cone. 



In addition to the elements which have already been described in the 

 ommatidium of Serolis, there are certain small pigment cells which oc- 

 cur for the most part in the region of the retinulse. Beddard ('84 a , 

 p. 21) describes these as long branching "connective-tissue cells," a 

 name which might imply that they originated from the mesoderm, and 

 were therefore intrusive. AVatase ('90, p. 293, Plate XXIX. Fig. 1) has 

 also described and figured these cells, but distinctly states his belief that 

 they are reduced ectodermic cells. In the adult I have observed in the 

 region of the cones, as well as near the retinulre, certain small nuclei 

 which are usually surrounded with more or less black pigment. These, 

 I believe, represent the cells described by Beddard and "vYatase. In the 

 embryo certain scattered nuclei (nl. h'drm., Figs. 65 and 70) occur in 

 the spaces between the ommatidia. It is probable that these nuclei are 

 ectodermic in origin, and I am at a loss to know what has become of 

 them in the adult, unless they form the pigment cells already men- 

 tioned. I am therefore inclined to believe, with Watase, that the small 

 additional pigment cells are reduced ectodermic cells. 



The presence of the hyaline cells in the ommatidium of Serolis is, as 

 Beddard has pointed out, almost a unique feature. These cells, usually 

 two in each ommatidium, till the space immediately below the rhabdome. 

 They are bladder-like (Fig. 62, cl. hyl.) and contain each a large gran- 

 ular nucleus. Although it is stated that there are usually two of these 

 cells in each ommatidium, I never found more than one to an ommatid- 

 ium in the several eyes of S. Schythei which I examined. This circum- 

 stance, however, is not surprising ; for, as Beddard ('84 a , p. 22) has 

 remarked, the number of these cells is subject to variation, there being 

 sometimes one, sometimes two, for each ommatidium. In S. Schythei 

 the single hyaline cell envelops more or less completely the distal part 

 of the fibrous portion of the cone cells, so that this part seems to pierce 

 the hyaline cell. A closer inspection, however, will usually show two 

 lines extending from the fibre to the periphery of the hyaline cell (com- 

 pare Fig. 62), and these lines indicate, I believe, the two walls of the 

 cell which have been infolded by the presence of the fibre during the 

 growth of the hyaline cell. 



The source of the hyaline cells is not definitely known. Their nuclei 

 (Fig. 65, nl. hyl.), as Beddard ('88, p. 450) has observed, are present 



