PATELLA. 235 



tentacles is probably of use as a protection, like that on 

 tlie visceral bump. 



The Eyes are a pair of organs situated on the outer 

 sides of the swollen bases of the tentacles (figs. 4 and 25), 

 but their shape and position does not seem to be affected 

 by the expansion, contraction and motion of these organs. 



The eye is really a simple pit, lined by a continuation 

 of the general surface epithelium, part of which has 

 become modified in connection with the sense of light- 

 perception. This modified epithelium is found over a 

 shell-like area, which approaches nearer to the opening 

 .of the pit on the lower and outer than on the tentacular 

 and central side (fig. 25). In this modified epithelium 

 we find two kinds of cells (fig. 26) : — (1) Elongated sensory 

 cells, with swollen bases, and long clear processes directed 

 towards the surface; and (2) long Pigment cells sur- 

 rounding and filling up the interspaces between the 

 sensory cells. Each cell consists of (a) an internal 

 tapering region, fairly clear, and containing a nucleus 

 in its outer part; (b) a broader middle region crowded 

 with minute pigment-granules, and (c) a clear outer 

 region. The Pigment is black and resembles that of 

 the mantle. 



Xerve fibres, going to the optic nerve, are foun^ 

 beneath this sensory epithelium. The cuticle is well 

 developed over the epithelium of the sensory region. The 

 eye of the Monobranch Docoglossa is more highly 

 developed, its cavity is filled with a jelly-like substance, 

 and the opening is a narrow slit. It is, therefore, almost 

 certain that the eye of Patella is degenerate, and this is 

 what might be expected from the conditions of life, since 

 the head remains under the shade of the conical shell. 



The Otocyst, like the eye, first appears as a depression 

 in the epidermis at the side of the head. With further 



