198 METCALF AND JOHNSON. 



These large cells lie among the smaller ganglionic cells of the 

 peripheral layer. A large bundle of nerve fibers, arising in the 

 fibrous core of the ganglion, passes through the peripheral cellu- 

 lar layer, just anterior to the protuberance on the posterior sur- 

 face, and enters the eye at its posterior end, forming the optic 

 nerve (Fig. 5, oil]. Fibers ramify also among the cells of the 

 postero-dorsal protuberance (Fig. 5, />/'). 



Four kinds of cells are found in the large eye, and one of 

 these, so far as known, is peculiar to this species. The pigment 

 cells and rod-cells resemble the characteristic cells found in the 

 well-developed eyes throughout the genus. The pigment cells 

 cover the dorsal surface of the eye from near the tip to slightly 

 beyond the middle (Fig. I and Fig. 5, tip). They then extend 

 around the lateral surfaces in posteriorly-directed broad bands, 

 uniting on the ventral surface to form a pigment layer which 

 extends from beyond the middle of the eye to the base (Fig. 5, vp). 

 The position of the dorsal and ventral pigment is also shown in 

 cross sections (Fig. 7, dp ; Fig. 9, vp). A cross section through the 

 middle of the eye (Fig. 8), shows the complete enwrapping of 

 this portion of the eye by pigment. The pigment cells lie out- 

 side the optic membrane (Fig. 5, <?,?), which is a continuation of 

 the ganglionic membrane. They are therefore probably meso- 

 dermal. 



The rod-cells are elongated cells with large nuclei (Fig. 5, dr 

 and vr). They have thick-walled basal ends and thin-walled 

 tips. In the anterior and posterior regions of the eye the thick- 

 walled ends of the rod-cells are toward the pigment (Figs. 5, 7 

 and 9). In the center of the eye, where the pigment forms a 

 continuous enveloping layer, the thick-walled inner ends of the 

 rod-cells have a somewhat confused arrangement, while their 

 thin-walled tips point toward the pigment cells (Figs. 5 and 8). 



The third kind of cells present in the eye are similar to the 

 ordinary cells of the ganglion, having nuclei of about the same 

 size. These cells, which have been called intermediate cells, lie 

 between the thick-walled ends of the anterior and posterior rod- 

 cells and the pigment cells (Fig. 5, / and i' ; Fig. 7, i ; Fig. 9, i'). 

 Intermediate cells are not present in all species. In the chain 

 forms of Cyclosalpa pinnata and Pcgea scittigcra-confedcrata the 

 rod-cells abut directly on the pigment layers. 



