148 



THE EYE IN EVOLUTION 



Pecten 



completely different. The receptive ends of the visual cells lie upon 

 an ectodermal layer of pigment cells crowned by a reflecting tapetum, 

 while their distal ends are prolonged as nerve fibres which run over 

 the retina towards the periphery and then bend backwards circum- 

 ferential ly to form the optic nerve which issues posteriorly. 



The eye of Pecten is of umisual interest (Fig. 123) ^ ; that of Spondylus is 

 similar.^ A single layer of epithelial cells forms the cornea, underneath this is a 

 clear cellular lens, and posteriorly, separated from the lens by a transverse 



Figs. 122 and 123. — Inverted Retina in Molluscs. 



Fig. 122. — The dorsal eye of Onchidium. 

 Showing an inverted retina pierced by 

 the fibres of the optic nerve, resembling 

 the arrangement in Vertebrates. 



CC, connective tissue forming the cor- 

 nea ; Ep, epithelium ; F, fibrous tissue 

 capsule ; ON, optic nerve ; ONF, optic 

 nerve fibres ; P, pigment layer of the 

 retina ; R, visual cells of the retina ; 

 V, two large vitreous cells (after Glad- 

 stone). 



O.N. 



Fig. 123.— The eye of Pecten. 



C, cornea ; Ep, surface epithelium ; 

 G, ganglion cell layer of the retina ; 

 L, cellular lens ; ON, optic nerve ; 

 P, layer of pigmented cells and above it, 

 the tapetum ; R, layer of rods ; V, 

 cavity of the vesicle ; VS, vascular 

 sinus (after Hesse). 



Spondylus 



septum, lies the flattened optic vesicle, the cavity of which has become virtual. 

 The retina itself is complicated. The proximal (deep) portion of the vesicle 

 consists of a single layer of cubical pigmented cells covered by a tapetum ; the 

 more superficial portion of the vesicle consists of two well-defined layers — a 

 proximal layer of rod-like visual elements, the receptive ends of which point 

 posteriorly into the cavity of the vesicle, and a distal layer of cells (the ganglion 

 cell layer of Patten, 1886) through which pass nerve fibres from the visual cells 

 as they run towards the periphery at the equatorial region whence (as in Cardium) 

 they encircle the posterior part of the globe to form the optic nerve (Kiipfer, 



' ^ee Keferstein (1862), Patten (1886). Kalide (1888), Carriere (1889), Schreiner 

 (1896). .;psse (1900-2). 

 2 ■■ Hickson (1882). 



