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", 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 (nd. px.), and differ consider- 
ably from those of the accessory pigment cells (n/. hidrm.). 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 retinule 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 Spheroma, 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. 94), and Palemonetes (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- 
