ORGANS OF VISION. 473 



cuticular lens is placed in the cavity, on the side adjoining the 

 cornea. This eye originates from the ectoderm, within the velar 

 area, and close to the supra-cesophageal ganglia, usually al the 

 base of the tentacles. According to Rabl (Vol. II. No. 268) it is 

 formed as an invagination, the opening of which soon closes ; 

 while according to Bobretzky (Vol. II. No. 242) and Fol it arises 

 as a thickening of the epiblast, which becoming detached takes 

 the form of a vesicle. It is quite possible that both types of 

 development may occur, the second being no doubt abbreviated. 

 The vesicle, however formed, soon acquires a covering of pigment, 

 except for a small area of its outer wall, where the lens becomes 

 formed as a small body projecting into the lumen of the vesicle. 

 The lens seems to commence as a cuticular deposit, and to grow 

 by the addition of concentric layers. The inner wall of the vesicle 

 gives rise to the retina. 



The most highly differentiated molluscan eye is that of the 

 Dibranchiate Cephalopoda, which is in fact more highly organized 

 than any other invertebrate eye. 



A brief description of its adult structure 1 will perhaps render more clear 

 my account of the development. The most important features of the eye 

 are shewn in fig. 277 C. The outermost layer of the optic bulb forms a kino) 

 of capsule, which may be called the sclerotic. Posteriorly the sclerotic abuts 

 on the cartilaginous orbit, which encloses the optic ganglion (G. op) ; and in 

 front it becomes transparent and forms the cornea Co, which may be either 

 completely closed, or (as represented in the diagram) perforated by a larger 

 or smaller opening. Behind the cornea is a chamber known as the anterior 

 optic chamber. This chamber is continued back on each side round a 

 great part of the circumference of the eye, and separates the sclerotic from a 

 layer internal to it. 



In the anterior optic chamber there are placed (i) the anterior part of 

 the lens (/*) and (2) the folds of the iris (Ir), The whole chamber, except 

 the part formed by the lens, is lined by the epidermis (Int 1 and 7/ 2 ). 

 Bounding the inner side of the anterior optic chamber is a layer which is 

 called the choroid (Int 1 } which is continued anteriorly into the fold of the 

 iris (Ir). The most superficial layer of the choroid is the epithelium already 

 mentioned, next comes a layer of obliquely placed plates known as the 

 argentea externa, then a layer of muscles, and finally the argentea interna. 

 The argentea interna abuts on a cartilaginous capsule, which completely 

 invests the inner part of the eye. 



The lens is a nearly spherical body composed of concentric lamellae of a 

 structureless material. It is formed of a small outer (7 1 ) and large inner 



1 Vide Hensen, Zeif. f. wiss. Zool. Bd. XV. 



