MUSEUM OF COMPARATIVE ZOOLOGY. 119 



the cones. The way in which this differentiation may have occurred 

 has already been suggested in my paper on the lobster ('90 a , p. 57). 



Although I have expressed the opinion that these cells are to be re- 

 garded as modified retinular cells, it might be maintained that they are 

 merely enlarged accessory pigment cells, such as occur in the inter- 

 ommatidial space of many Crustaceans. But I believe such an interpre- 

 tation of these cells would be erroneous, for the following reason. In 

 Serolis the nuclei of the pigment cells which surround the cone (Plate VI. 

 Fig. 65, nl. dst.) possess one, and sometimes two, well marked nucleoli, 

 but no fine chromatine granules. In this respect they closely resemble 

 the nuclei of the proximal retinular cells (jiLpx.), and differ consider- 

 ably from those of the accessory pigment cells (nl. h'dnn.). The nu- 

 clei of the last named cells contain only fine granules. So far, then, 

 as their nuclei are concerned, the distal retinular cells bear a much 

 closer resemblance to the proximal cells than to the accessory pigment 

 cells. Each retinula in Serolis contains, moreover, only four cells, and 

 in this respect differs considerably from other Isopods, where the number 

 of retinular cells is either six or seven. On the supposition that the 

 pigment cells surrounding the cone in Serolis are accessory pigment 

 cells, one would be called upon to account for the exceptionally small 

 number of cells in the retinula of this genus ; whereas, if the cells 

 around the cone are regarded as modified retinular cells, they may be 

 taken to indicate for Serolis a primitive retinula composed of six cells, 

 a number characteristic of the retinula; in other Isopods. This inter- 

 pretation of the condition of the retinula in Serolis is borne out by 

 what is known of the retinula in Sphseroma, where, it will be remem- 

 bered, a transition between the condition in Serolis and that in other 

 Isopods was distinctly indicated. 



In the Stomatopods, Schizopods, and Decapods, if my observations 

 are correct, there are no ectodermic accessory pigment cells. Conse- 

 quently, a comparison between these cells and what I have called the 

 distal retinular cells cannot be drawn. In Mysis (Plate VII. Fig. 73), 

 Gonodactylus (Plate VIII. Fig. 9-t), and Palsemonetes (Plate IX. Fig. 

 103), as well as in all other Decapods which I have examined, the resem- 

 blance between the nuclei of the retinular cells and those of the pigment 

 cells which surround the cone is as striking as in Serolis, and suggests 

 the origin of these cells from retinular cells rather than from any other 

 source. In Homarus, the pigment cells around the cone present a con- 

 dition of some interest in this connection. Each pigment cell is extended 

 proximally as a long fibre, which certainly reaches nearly to the base- 



