454 Benjamin Sharp 



which the celi is placed. As is seen in the figure ali the cells are not of 

 the same shape. There is one set (a) in which we have the pigmented 

 portion at the broadest part of the celi, the base in fact; the other part 

 of the celi tapers to the tunica. These cells possess a nucleus which ap- 

 pears ovai. In the other set, in which the nucleus is spherical, the pro- 

 portions are reversed, the broad base resting on the tunica, while the 

 apex is wedged in between the bases of the other set. I am not pre- 

 pared to say whether the cells of this set contain pigment , as they are 

 very small and the end that passes outward is so fine , that , although I 

 used a strong immersion lense of Zeiss I was unable to satisfy myself 

 fully on this point. 



On the top or external part of the celi there is seen a transparent 

 membrane (fig. 2 e) , which seems to be continuous over ali the pig- 

 mented cells. It is of a homogeneous nature, highly refractive , and 

 takes no color in either picro- or borax-carmine. When the epithelium 

 is separated from the underlying cutis , and the cells that compose it 

 are a little scattered, this transparent membrane or cuticle, if it may be 

 so called remains intact , that is , each celi is broken off from it leaving 

 it unbroken. It may be seen projecting beyond the cells, which are 

 represented in the figure (fig. 2 e). 



On those cells possessing a cuticle no cilia could be detected. A 

 cuticle is probably formed by each celi and then these separate pieces 

 become welded together so as to form a continuous whole. 



Fraisse 1 found something similar to this in the eye of Fissurella, 

 that is to say , he found non - pigmented cells between the pigmented 

 ones and supposed these latter to form the lense and vitreous humor. 

 But in Ostrea I am inclined to believe that each celi produces the 

 cuticle which o verlies it. 



Now it is my opinion that the seat of vision is situated in these 

 pigmented cells with a highly refractive cuticle. This view is supported 

 by the fact that similar cells are found in Patella by Fkaisse' where 

 no lens whatever is present. This point will be considered 

 later on. 



I may cali attention bere to the so -called epidermis (really 

 cuticle, fig. 1 cu.) of the shell passing in between the external fold a 

 and middle fold b. This membrane is formed by the cells lining the 

 internai side of the external fold. 



1 P. Fraisse , über Molluskenaugen mit embryonalem Typus. Zeitschr. f. 

 wiss. Zool. Bd. XXXV. Leipzig 1881. p. 461. 



