62 W. J. DAKIN. 



details, somewhat severely. The epithelium covering the 

 eye-stalk is a direct continuation of the pallial epithelium, 

 but is modified in various regions of the eye-stalk and 

 becomes a transparent cornea over the free pole. Below the 

 optic vesicle the cells are small and cubical, or rather deepei* 

 than wide (PI. 6, fig. 1). They contain no pigment here, 

 and the nucleus is situated near the base. A distinct cuticle 

 is present. Some little distance below the optic vesicle these 

 cells increase in depth and at the same time begin to contain 

 pigment. This pigment extends further down that side of 

 the eye which is uppermost (see fig. 1, PI. 6 ; the right-hand 

 side is the shell side of the eye and also the uppermost, since 

 it is an eye from the left valve). At the level of the middle 

 of the optic vesicle, that is, about the plane of the septum, 

 the epithelial cells have attained their greatest depth and are 

 almost filled with dark pigment, occurring in the form of fine 

 granules. The external portion of the cells is usually less 

 thickly crowded, and if the sections are stained to bring out 

 the nuclei it will be seen that these have moved, with the 

 acquisition of pigment, so that they reside near the surface 

 instead of at the basal end. The statements of Rawitz and 

 Schreiner in regard to the colour of this pigment in the 

 different species appear to me to be of little importance, and 

 in any case I can hardly confirm them. The colour of the 

 o-ranules in Pecten iacobteus, P. maximus, and P. 

 opercularis is dark brown, and the exact shade varies in 

 any one species and according to fixation and preservation; 

 moreover, the cells are completely filled in P. jacobaeus, or 

 at least those of the upper side of the eye-stalk. 



Another point that may be noted here is that the increase 

 in height of the epithelial cells opposite the optic vesicle is 

 common to all the species I have examined, though Rawitz 

 states that in P. jacobasus the epithelium is everywhere the 

 same in height and figures it as such ([25], p. 106). Pecten 

 abyssorum possesses (Schreiner) no pigment in the cells of 

 the mantle-edge or of the eye-stalk. Patten appears to be 

 the only one who has noticed that there is more pigment 



