114 BULLETIN OF THE 



which perhaps pass through the basement membrane. In Palsemonetes 

 (Plate IX. Fig. 108, cl. 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 Palaemonetes (Plate IX. Figs. 103 and 109, nl. dst.) the 

 nuclei are grouped distinctly in pairs, one pair for each ommatidium. 



Each cone in Penteus, 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 

 vhich supports Patten's statement concerning the four pigment cells in 

 Penseus. I am therefore inclined to doubt the accuracy of these two 

 observations. 



The interommatidial space in the basal part of the retina in Palse- 

 monetes contains a light pigment similar to that described in the retina 

 of Mysis. Like this the pigment in Palaemonetes is white by reflected 

 light, and yellowish by transmitted light (compare Plate IX. Fig. 115). 

 It is apparently contained within cells (Fig. 103, cl. msdrm.) 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. Tliey have been 

 seen by Carriere ('85, p. 169) in Astacus, by Patten ('86, p. 636) in 

 Peneeus, 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 Palaemonetes. 



From what has preceded itns evident that the ommatidium in Deca- 

 pods contains the following elements : cells of the corneal hypodermis, 



