Eyes of Molhises and Arthropods. 549 



The pigmeuted areas culminate iu a slight elevation, on the suui- 

 mit of which the eyes are usnally situateci; they either project some 

 distance above the surface, or occupy shallow depressions, where they 

 hardly more thau attain the level of the surrounding cpithelium. In all 

 cases ; they are surrounded by a narrow and less deeply colored furrow 

 (figs. 44 and 45). 



The invaginated eyes are smaller, and more easily overlooked. 

 than the faceted ones. They form a narrow band along the summit of 

 that portiou of the Ophthalmie fold, beueath the ventral opening in the 

 Shell through which the byssus projects. A certain amount of light can 

 always penetrate this opening, which cannot be closed, and is only pro- 

 tected by a brush-like outgrowth of the cuticular coveriug of the shell . 

 These eyes are perfectly black, round or oval, pigmented cups varying 

 in size from .014 to .07 mm in diameter. Sometimes they form the 

 floor of shallow, saucer-like depressions of the epidermis, or, as in the 

 more perfected forms, lie at the bottom of deep pits, whose openings 

 have often become coustricted into narrow slits, extending at right 

 angles to the edge of the mantle. In the oldest examples, the pits are 

 less numerous, or often absent, on the anterior portion of the mantle, 

 where the faceted eyes are most abundant : in the middle, where the 

 faceted eyes are absent, they reach their highest development. In the 

 posterior portion there are a few faceted eyes and numerous invaginated 

 ones of all sizes and degrees of development, so that in many cases it 

 is difficult to teil whether they belong to one type, or the other. The 

 arrangement described above is that generally met with, and although 

 there is a wide individuai Variation, one is always sure to find certain 

 areas occupied by both faceted and invaginated eyes, together with 

 numerous, intermediate forms (figs. 46 and 47). The larger, and more 

 highly developed, invaginated eyes are arranged in a single row along 

 the summit of the Ophthalmie fold, often so near together as to touch 

 each other : while, on each side, but more especially on that next the 

 velum, are inuumerable, smaller eyes showing ali gradations, from 

 simple pigmented spots, or even Clusters of two or three pigment cells, 

 to the fully formed eyes. 



The mantle edge in this region is colorless, with the exception of 

 pigmented areas similar in size and shape to those in which the faceted 

 eyes are situated. The number of these pigmented areas is much 

 smaller than that of the eyes , several of which are often situated iu 

 one of these spots. A network of shallow canals, formed by simple 

 folds of the skin, is distributed over the edge of the mantle. The invag- 



