114 BULLETIN OF THE 
which perhaps pass through the basement membrane. In Paleemonetes 
(Plate IX. Fig. 108, ef. dst.) and in Cancer (Plate X. Fig. 127, cl. dst.) 
they are reduced to pigmented threads, which, starting from comparatively 
large bases, twine around the lateral surfaces of the cones. 
The arrangement and number of the distal retinular cells can be most 
readily determined from their nuclei, In Cancer (Plate X. Fig. 128) 
the cells are arranged in circles of six around each group of cone cells ; 
each cell, however, participates in three circles, and consequently there 
are in reality only twice as many cells as ommatidia. This arrangement 
of the cells also occurs in Cardisoma, Hippa, and Pagurus. In Crangon 
(Fig. 123), as I have previously remarked, the nuclei of the distal retinu- 
lar cells are arranged in rows alternating with the rows of cones. There 
are twice as many nuclei as cones; hence I conclude that here also 
there are two distal cells for each ommatidium. In Homarus, Palinurus, 
Cambarus, and Palemonetes (Plate IX. Figs. 103 and 109, nl. dst.) the 
nuclei are grouped distinctly in pairs, one pair for each ommatidium. 
Each cone in Penzus, according to Patten (86, p. 634), is surrounded 
by two pairs of pigment cells, and Watase (’90, p. 299) states that in 
Cambarus the dioptric part of the ommatidium is sheathed by four pig- 
ment cells. In Cambarus Bartonii I have been able to find only two 
such elements, the pair of distal retinular cells already described, and in 
the other Crustaceans which I have studied I have observed nothing 
which supports Patten’s statement concerning the four pigment cells in 
Peneus. I am therefore inclined to doubt the accuracy of these two 
observations. 
The interommatidial space in the basal part of the retina in Pale- 
monetes contains a light pigment similar to that described in the retina 
of Mysis. Like this the pigment in Palemonetes is white by reflected 
light, and yellowish by transmitted light (compare Plate IX. Fig. 115), 
It is apparently contained within cells (Fig. 103, cl. ms’drm.) whose out- 
lines are very irregular, and whose nuclei (Fig. 104, nl. ms’drm.) are 
small and somewhat variable in form. These cells occur on both sides of 
the basement membrane. As in Mysis, they have probably migrated 
into the retina, and are perhaps mesodermic in origin. They have been 
seen by Carriere (’85, p. 169) in Astacus, by Patten (86, p. 636) in 
Penzeus, and by myself (’90*, p. 25) in Homarus. I have also recently 
observed them in Crangon, Cambarus, Cardisoma, Pagurus, and Pali- 
nurus, as well as in Palemonetes. 
From what has preceded it is evident that the ommatidium in Deca- 
pods contains the following elements: cells of the corneal hypodermis, 
