Eyes of Molluscs and Arthropods. 749 



sections, where they are particularly striking; they are often sunken be- 

 low the surtace to form minute pits. 

 Fig. 47: Represents a uearlycolorless portion of the mantleedge oiArca barhatu, 

 oontaining two faceted eyes and a nearly closcd invaginate one. Aroiind 

 the eyes one sees numerous colorlcss cells, some of which are doublo, 

 surrounded by pigment. They represent incipient ommatidia. 

 Fig. 48. A detached, double rod from the compound eye of Area Noae (macerat- 

 ed in chromic acid Vso^)- The division between the two halves ol' the 

 rod, r/t.i and r/(.2, is well marked. To the sides of the rod are attachod 

 numeroua nerve fibres from which are given off many lateral branches 

 that cling very tenaceonsly to the surface ; the free portions of the fibres 

 are covered with innumerable fibrillae. In this example the loops of the 

 external nerve fibres were not visible. 

 Fig. 4',i. A detached pigment cell, with its rod, showing the large, basally situated 

 nucleus , the root-like prolougations of the cells, and the external nerve 

 fibres, ex.n. 



Fig. 50. A colorless sense cell (incipient retinophora?) from the epithelium of 

 Pectunmlus. The cuticula is very slightly developed and can hardly be 

 Said to form a rod; the nucleus is nucleolated and the base of the cell is 

 drawn out into a single, long fibre. A nerve fibre, n.f., extends along the 

 wall of the cell, and is free everywhere except at its outer end where 

 it is firmly attached to the cell wall. 



Fig. 51. A pecuiiar, hyaline structure, which Ihave observed several tinies among 

 the isolated cells from the eyes and Ophthalmie fold of Area Ni>ae ; I can- 

 not offer any Suggestion as to its nature or origin. 



Fig. 52. A nearly fresh retinophora (macerated for twenty four hours ìuMììllers 

 fluid) from the compound eye of Area Noae. It is a very instructive ex- 

 ample, in that it sliows with great clearness that the retinophora is com- 

 posed of two cells , the outer ends of which remain ununited ; the rods 

 belonging to the halves are likewise perfectlydistinct; in each lialf of the 

 cell is a nucleus, that marked ii.rf.- is the smaller , and more difficult to 

 stain. 



Fig. 53. An isolated pigment cell from the general epithelium oi Pectuìiculus; two 

 long and comparatively large, nerve fibres are attached to the outer end 

 of the cell. 



Fig. 54. A section through one of the pseudo-lenticulate eyes of ^jt« Naae. In 

 this example the retineum is quite deeply invaginated ; in other cases 

 it is perfectiy flat or nearly so. 



Fig. 55. Section through the mantle edge of Pectunculus . 



Fig. 50. Section through the mantle edge of Area Noae, showing the prominent Oph- 

 thalmie fold, o.f., lipon the summit of which is situated a compound ej'e. 



Fig. 57. A section of the mantle edge oi Area barbata , showing a portion of the 

 epithelium, highly magnified, with the r e ti a t e r m i n a 1 i a of hypoder- 

 mic nerve fibres. 



Fig. 5S. Section through the eye of Ilaliotis. The double layered cuticula has, 

 over the retineum, divided into two very distinct layers; an inner 

 one, rh., divided into distinct segments, or rods, containing a highl}^ de- 

 veloped part of the retia terminalia; and a second, outer layer, formed 

 by the thickened corneal cuticula; the corneal layer is divided into two 

 secondary parts, a semi-fluid inner portion, thevitreous body, v.b., and a 



