466 SHAFER 



of the retinula this seventh cell, which is entirely surrounded at 

 its distal end, is found squeezed out between the other 6 retinular 

 cells and is not here completely surrounded by them (Fig. 9, £). 

 This condition holds for the retinula for its entire proximal half. 

 It is true also that this seventh cell crops out in every case on 

 the same side of the retinula, namely, on that side of the re- 

 tinula turned toward the inner ventral angle of the eye. Fig. 

 9, D, shows 3 adjacent retinulae in cross-section in the region 

 of the nuclei. These nuclei are long-elliptical in shape (Fig. 

 9, A, rn), and in cross-section they are not all the same size, 

 since some are cut near the middle and some near their ends. 

 In the cross-section of every retinula, however, the nucleus of 

 the narrow seventh cell may be found near its outer margin 

 (Fig. 9, Z>, 7«). The rhabdomeres are all smaller at the prox- 

 imal end of the retinula, but they are always 7 in number, the 

 odd one occupying the axial position at the inner part of the 

 narrow seventh cell. These facts, taken with that of the con- 

 stant presence of the seventh nucleus, make it certain that this 

 peculiar seventh structure is truly a retinular cell whose distal 

 end is entirely surrounded by the corresponding ends of its 6 

 companions. The proximal ends of the retinulae are bounded 

 by a very thin basement membrane, bm (Figs. 9, A, and 10). 

 A little beneath this membrane spreads a somewhat thicker 

 granular tapetum, tp (Figs. 9 and 10), and immediately under 

 this is a network of tracheae, tr. Leading from the inner prox- 

 imal end of each retinula through the basement membrane, the 

 tapetum, and between the tracheae is a narrow bundle of nerve 

 fibers, which are soon lost in a fine granular layer, gr (Figs. 

 9, A, and 10), just within the trachial network. 



The iris pigment of the large element part of the eye is com- 

 paratively slight. It is contained in narrow pigment cells, nsp 

 (Figs. 9 and 10), which surround the pseudocones and extend a 

 little way down between the retinulae. Fig. 9, C, sip shows 

 the arrangement of these cells between the retinulae. The 

 proximal three fourths of the retinulae have no pigment cells 

 around them at all and the retinulae themselves touch each other 

 (Fig. 9, D). 



The conditions described above also hold for the small eye 



