MUSEUM OF COMPARATIVE ZOOLOGY. 81 



the sections which pass through the retina. The most ventral section 

 is shown in Figure 20, the most dorsal in Figure 29. 



Immediately below the lens the central part of the retina is occupied 

 by a roundish granular mass (Fig. 18, con.), which in the living animal 

 is the only part without pigment. In trausverse sections this mass is 

 seen to consist of two bodies (cl. con. 1, and cl. con. '2, Fig. 25), which 

 extend as far as to the lens (compare Figs. 25-27). Each body con- 

 tains a nucleus (nl. con., Figs. 25 and 27) and consequently represents 

 a cell. From the position which the mass occupies, and from the fact 

 that it contains no pigment, it represents, I believe, a cone, and the two 

 cells of which it is composed are its two segments. 



Claus ('63, p. 47) states that in Pontella each retina is provided with 

 six or more small crystalline cones, but my own observations do not 

 confirm this statement. The body which, on account of its position, I 

 have described as the cone in Pontella, is probably homologous with 

 what Dana ('50, p. 133) first described as the inner lens in Corycseus, 

 and with what subsequent investigators have called the crystalline cones 

 in Sapphirina (Gegenbaur, '58, p. 71) and Copilia (Leuckart, '59, p. 252). 

 ^Nothing, I believe, is known of the cellular composition of the cone in 

 these genera. 



The arrangement of the elements in that portion of the retina which 

 surrounds the cone in Pontella is not easily made out. The most con- 

 spicuous structures in this region are rod-like bodies, which probably 

 represent rhabdomeres. Eight of these, arranged in three groups, are 

 present in each retina. The largest group, composed of five rods, lies 

 directly beneath the cone. The rods of this group have been numbered 

 from one to five in the retina to the left in Figures 21, 22, and 23. 

 Posterior to this group, in the same retina, is the sixth rod, seen in 

 Figures 24, 25, and 26. Anterior to it are the seventh and eighth 

 rods, seen in Figures 26, 27, 28, and 29. 



The outlines of the cells to which these rods belong cannot alwavs be 

 distinguished ; that there is a cell for each rod is evident from the fact 

 that near each rod there is a large nucleus. The nucleus belonging to 

 the cell from which the eighth rod was produced is shown in Figure 28 

 (id. rtn.') ; those belonging to the cells from which the sixth and seventh 

 rods arose are indicated in Figure 26 (nl. rtn'.), and those belonging to 

 the cells from which the central group of five rods came are seen, four in 

 Figure 24 and one in Figure 25 (nl. rtni). 



In addition to these nuclei, which judging from their positions and 

 number are unquestionably the nuclei of the cells to which the rhab- 



vol. xxi. — no. 2. 6 



