No. I.] EYES OF MOLLUSCS AND ARTHROPODS. 73 



The faceted eyes, of which there are about two hundred in 

 each individual, are the most highly developed of all ; and they 

 are of special interest, since they possess all those characters 

 which distinguish the so-called compound or faceted eyes of 

 Arthropods. The faceted eyes o( Area are collections of about 

 eighty highly specialized ommatidia to form minute hemispheri- 

 cal projections, lineally arranged along the summit of the 

 ophthalmic fold, at the anterior and posterior portion of the 

 mantle edge. In these eyes, the colorless cells, or retinophorcB, 

 are quite large (Fig. 8), and contain two nuclei, and an axial 

 nerve-fibre ; on their outer ends is a large, double rod which 

 projects slightly above the surface. Carriere mistook these re- 

 fractive rods for minute lenses. The inner part of the conical 

 retinophora is filled with a mass of brilliantly refractive glob- 

 ules which act as reflectors, causing the light to pass a second 

 time through the rods. Both nuclei are situated at the outer end 

 of the retinophora ; one is large, stains deeply, and contains a 

 well-marked nucleolus («. r/y) ; the other («. rf?'), is usually 

 smaller, and seldom absorbs coloring matter. 



One often finds, however, cases in which both nuclei are quite 

 alike, while the outer end of the cell is strongly bifurcate, prov- 

 ing beyond doubt that the retinophora is formed by the fusion 

 of two cells. Each retinophora is surrounded by eight retin- 

 ulse, arranged in two circles of four cells each. In one circle 

 the four retinulcE are pigmented only at their inner ends, which 

 form a complete sheath for the inner ends of the retinophorae {pg.^) 

 The outer third of each cell is reduced to a very thin and colorless 

 membrane, which unites with the similar prolongations of the 

 three other cells to form a delicate sheath around the outer ends 

 of the retinophorae. In the other circle (j>g}), the inner ends of 

 the four retinulae are reduced to slender and colorless stalks, or 

 bacilli, (d.c}) while their swollen outer ends form a complete 

 sheath of pigment around the double rod. It is important to 

 notice that the retinulae of the evaginate eye have lost their rods, 

 and now serve simply as a covering to exclude lateral rays of 

 light from the highly developed rods of the retinophorae. The 

 first step towards the formation of the eyes in Area is the collec- 

 tion of the isolated ommatidia into groups. At the same time 

 there is a thickening of the retinidial cuticula, to form over each 

 cell a cuticular column, or rod, which contains a part of 



