MUSEUM OF COMPARATIVE ZOOLOGY. 113 
‘of these nuclei affords no clue as to which one belongs to the rudi- 
mentary cell. 
In Palinurus (Plate X. Fig. 125, nl. pa.), the eighth nucleus is regu- 
larly present and easily seen. In Cancer (Fig. 129, nl. px. 8) it occu- 
pies a position between the adjacent retinule. It can also be identified 
in Crangon. 
The retinula in Decapods, according to all recent observers, contain 
seven functional cells. In Homarus, Palinurus, Cambarus, Crangon, 
Paleemonetes, and Cancer, the retinule contain, in addition to the 
seven nuclei of the functional cells, an eighth nucleus, which repre- 
‘sents, I believe, a rudimentary cell. It is probable, therefore, that in 
all Decapods each retinula really contains eight cells, one of which is 
rudimentary. 
The rhabdome in Decapods presents a very uniform structure. It is 
usually an elongated body, pointed both at its distal and its proximal end, 
and completely covered, except at its distal tip, by the proximal retinular 
cells. In those Decapods in which it is large enough to be conveniently 
observed, its transverse section is squarish, and usually subdivided by 
two straight lines into four smaller squares (Plate IX. Fig. 113). As 
Grenacher (77, pp. 31, 32) first demonstrated in Palemon, the retinular 
cells are rather peculiarly arranged around the rhabdome. One of its 
four sides is flanked by one cell, the other three by two cells each. This 
arrangement can be seen in Palzemonetes (Fig. 113), and probably obtains 
for all Decapods. 
In Palinurus Argus (Plate X. Fig. 124) there appears to be no rhab- 
dome, unless the translucent axial portion of each retinular cell can 
be said to represent segments of it. The fibrous ends of the cone cells 
(cl. con.) can be easily identified between the retinular cells, but the 
centre of the retinula is filled with pigment, and shows not the least trace 
of a rhabdome. This peculiarity of Palinurus was noticed as early as 
1840 by Will (40, p. 15), who described the ommatidium in this genus 
as being without a transparent mass (= rhabdome). 
Although the distal retinular cells in Decapods were collectively rec- 
ognized by Miiller (26, pp. 355, 356) some sixty years ago as a definite 
pigment. band in the distal portion of the retina in the crayfish, they 
were not identified as separate cells until quite recently. The first in- 
vestigator to observe them was Carriere (’85, p. 169), who described 
them in Astacus as a pair of pigment cells flanking each cone. In Cam- 
barus, Crangon, and Homarus, they also cover the sides of the cone, and 
in the last named genus they are produced proximally into long fibres, 
VOL. XXI.—NO. 2. 8 
