MUSEUM OF COMPARATIVE ZOOLOGY. 107 



The rhabdome (Figs. 94 and 95, rhb.) in Gonodactylus is an elongated 

 rod-like structure of uniform thickness, which extends from the region 

 of the proximal retinular nuclei to the basement membrane. It shows 

 a distinctly toothed edge (Fig. 94), especially in specimens which have 

 been treated with potassic liydrate. In transverse section it is squarish. 

 Owing to its small size, the exact relation of the seven surrounding cells 

 to its four faces cannot be easily determined. The single unpaired cell 

 (Fig. 101) certainly lies opposite a face, not an angle. In this respect 

 it agrees with the unpaired cell in Squilla as figured by Grenacher (79, 

 Taf. XI. Fig. 122). Probably in Gonodactylus the remaining six cells 

 are related to the sides of the rhabdome as the coiTesponding ones are 

 in Squilla (compare Grenacher's Fig. 122). In Gonodactylus the retinu- 

 lar cells and rhabdome are in close contact with one another. The 

 separation of these elements as figured by Grenacher in Squilla is prob- 

 ably artificial, as Grenacher himself suggests. In Squilla, according to 

 both Steiulin ('68, p. 17) and Grenacher ('79, p. 125), the rhabdome 

 in transverse sections is subdivided into four equal parts, somewhat as 

 in Mysis. I have not observed this condition in Gonodactylus. 



The distal retinular cells in Gonodactylus occupy the usual position 

 near the cones. They contain very little pigment, and their number 

 can be determined only by that of their nuclei. These agree with the 

 nuclei of the proximal cells in the possession of a single well defined 

 nucleolus, which is most readily seen in depigmented sections (compare 

 nl. dst. and 7il. px. in Fig. 94). The distal nuclei, especially in the 

 region of the larger ommatidia, are arranged in rows which alternate 

 with the rows of cones (Fig. 99, nl. dst.). Although the nuclei are not 

 very definitely arranged, they often show a tendency to be grouped in 

 pairs, and these pairs are so placed that in each row there is evidently 

 one for each adjacent ommatidium. Moreover, in equal lengths of ad- 

 joining rows of nuclei and cones, the nuclei are always double the num- 

 ber of cones. I am convinced by these facts that there are two distal 

 retinular cells for each ommatidium. 



Besides the cells already described, certain others occur in the proxi- 

 mal part of the retina in Gonodactylus. These are represented by a 

 few small, elongated nuclei (Fig. 94, nl. ms'drm.), which are very similar 

 in appearance to certain nuclei occurring in the spaces below the base- 

 ment membrane. I therefore believe that in Gonodactylus, as in Mysis, 

 the proximal portion of the retina is occupied by intrusive cells, which 

 are probably mesodermic in origin. 



The kinds of cells found in the ommatidium of Stomatopods are as 



