74 PATTEN. [Vol. I. 



the " retia terminalia" The corneal cuticula remains as a thin 

 membrane covering the outer ends or the rods. (Fig. 7.) If the 

 rods formed a lens-hke thickening over such a cluster of omma- 

 tidia there would be formed z. pseudo-lenticulate eye. If such 

 an eye were invaginated an optic cup would be formed ; if it 

 were evaginated, and the ommatidial cells slightly modified, one 

 of the faceted eyes would be the result. 



ArcaNocB, therefore, is a valuable subject for the study of the 

 origin of the eyes ; for there we have a complete series of transi- 

 tional forms between highly specialized visual cells and simple 

 epithelial ones ; but, what is of still more value, we have all 

 stages in the development of the nerve-fibres of the undiffer- 

 entiated epithelium up to those supplying the most specialized 

 visual cells. In Area we have conclusive proof that the so- 

 called rods of the eye are derived from cuticular thickenings 

 over sense-cells, and that the cuticula serves no other purpose 

 than as a support for a system of minute nerve-fibrillae, which are 

 the real sensitive elements of the eye. 



We have in Area a sluggish, and for the most part fixed, 

 animal lavishly supplied with over twelve hundred well-devel- 

 oped light-sensitive organs, not to mention the innumer- 

 able isolated ommatidia scattered everywhere over the sur- 

 face of the mantle. The presence of all these organs in such 

 an animal may well excite surprise especially when we consider 

 that Avicida, a related genus, and one that is not provided with 

 specialized eyes (so called), is exactly as sensitive, if not more 

 so, to changes in the amount of light as is Area. 



Pectunculus has about twenty-five faceted eyes, similar in 

 structure to those of Area, upon the right mantle edge, and 

 twenty-two on the left. No invaginate or pseudo-lenticulate 

 eyes are present. 



Anatomy of the Eyes of Pec ten. 



The eyes of Pecten, since Poli first described them as such, 

 in 1795, have attracted the attention of many zoological 

 students, not a few of whom have made them the object of 

 special study. The general structure of the organs in question 

 is, therefore, well known. 



I have shown that the control of the dioptric apparatus was 

 more perfect than had been supposed. The curvature of the 



