Fig. ;U). Sectiou through the tapetum , argentea and double layered sclerotica of 

 /'. Jacobaeus ; one sees the problematica! fibres (degenerate nerve 

 fibres?) with the spindle-shaped swelling in the sclerotica. The inner 

 layer of the argentea contains a degenerate nucleus. 



Fig. 40. A detached, peripheral piece of the septum of F. Jacobaeus, showiug the 

 fibrous-like projections upon its inner surface, «. 



Fig. 41. A Portion of the anterior mantle edge oi Area barbata, showing the com- 

 pound eyes situated in the triangulär, pigmented areas ; the latter also 

 contain a number of isolated ommatidia. Four of the eyes are united to 

 form two pairs. The single, as well as the double, eyes are situated in 

 slight depressions, represented by the light rings, upon the summits of 

 gentle elevations. 



Fig. 42. Scction through an invaginate eye of ^rc« barbata. The double-layered 

 cuticula c.c. and r.c. is continued over the retineum, where the retinidial 

 layer, /•. c, becomes greatly thickened to form the retinidial layer, r.c.^ 

 of the eye. This layer, /-.c.', is not distinctly divided into Segments, or 

 rods, for each cell. One may foUow the nerve fibres of the retinidial 

 layer between the cells, and through the basal membrane, b.m. Below 

 the retineum are several slime cells, sl.c. 



Fig. 43. Section through the Ophthalmie fold of Area barbata, showing a faceted 

 eye, ovit. , and immediately below it, an invaginated one. 



Fig. 44. A highly magnified eye o^ Area Noae. The dark circles represent the co- 

 lorless retinophorae, through which one sees the black pigment of the 

 inner row of cover cells. The refractive, lens-like bodies seen on the pe- 

 riphery, are the protuberant rods of the retinophorae. 



Fig. 45. Sectiou through the Ophthalmie fold and eye of Pcctunculus. One sees the 

 nuclei of the inner and outer row of retinulae, or cover cells, and the nuc- 

 leolated ones of the retinophorae, n.rf. The double rods of the retinopho- 

 rae, rh , are also seen, somewhat shrunken by the reagents , with their 

 inner faces represented either as simple , or double concavities. In this 

 instance some of the fibres had united to form a distinct optic nerve op.n. 

 Blood cavities are specially numerous in those parts of the Ophthalmie 

 fold where there are many eyes. 



Fig. 46. Surface view of an invaginated eye, in.e. , and of isolated ommatidia, 

 Olli., from the median portion of the mantle edge of Area barbata. The 

 invaginate eye is open and one sees several points of white light issuing 

 from the refractive and colorless globules in the retinophorae. At x. is 

 a group of three or four intensely black pigment cells, which, apparently, 

 do not contain a retinophora. One often finds such Clusters of cells in 



