ON THE STRUCTtJRE OF ENIGMA ^NIGMATICA. 281 



finely granular substance containing a few nuclei, and as the 

 latter are identical with the nuclei of the cornea, it is 

 probable that the lens is formed by a proliferation of the 

 epithelial cells. The lens is embedded in a vitreous body 

 consisting of a mass of finely granular, vacuolated proto- 

 plasm containing numerous nuclei which are smaller and stain 

 more deeply than the nuclei of the lens or cornea. This 

 vitreous body, in which no cell outlines are distinguishable, is 

 evidently a local modification and concentration of the sub- 

 epidermal tissue. The retinal layer, as is clearly shown in 

 fig. 20, is formed by the epithelium lining the inner face of 

 the mantle. Some of the eyes are borne on very thin parts 

 of the mantle, and in these, as the vitreous body occupies the 

 whole thickness of the mantle, the retina simply consists of a 

 modification of the cells lining the inner face of the mantle. 

 They become very large and columnar (fig. 21), and are thickly 

 loaded with black pigment granules, but there are no chitin- 

 ous rods or rhabdomes, such as are commonly found in 

 retinal cells. On the other hand they bear a close resem- 

 blance to the retinal cells of the visual organs of the siphons 

 ofMya arenaria, Solen vagina, andDreissensia poly- 

 morph a, figured by Sharp (17). In the case of those eyes 

 situated on thickened portions of the mantle, the internal epi- 

 thelium is deeply invaginated, as is shown in fig. 20, and the 

 extremity of the invagination spreads out below the vitreous 

 body to form a two-layered optic cup which in section bears 

 some resemblance to the optic cup of a developing vertebrate 

 eye. The cavity of the optic cup is in free communication 

 with the pallial cavity by the "optic stalk," if we may give 

 this name to the stalk of invagination. In most cases the 

 whole of the invaginated cells are pigmented, those of the 

 stalk as well as those of the cup, and both layers of the cup 

 are always pigmented, but it is only the cells of the anterior 

 layer of the cup, that is to say, those in contact with the 

 vitreous body, that are enlarged and columnar. The hinder 

 wall of the cup and the walls of the stalk are composed of 

 low cubical or flat epithelial cells and the transition between 



