97 



left upper valve are the more uumeroiis, are larger and 

 situated on longer stalks in P. ma.viinus. This may be a 

 result of the asymmetry of the animal, and I have 

 observed that the left mantle lobe is frequently extended 

 so far out that it overlaps the shell edge, and the eyes are 

 directed horizontally out, or even upwards, in place of 

 downwards towards the ground, as has sometimes been 

 stated. The eyes vary in size considerably. There is a 

 group of large eyes anteriorly and posteriorly on each 

 mantle lobe. 



In P. opercularis there are about five eyes in each 

 such group, and separated by an interval from 

 these we have a series of eyes in the ventral 

 margin. Those nearest the interval are the smallest, 

 and the largest are to be found mid-ventrally. I do 

 not find, however, the regular arrangement of large 

 and small eyes described by Patten (35). Xeither have I 

 been able to find in P. opercularis any eyes '' the pupils of 

 which are entirely covered with pigment " (Patten, 35). 



The eye stalk (fig. 29, E. st.) which supports the 

 eyes is a short column, and is retractile, but one does not 

 observe the eyes being turned in diflterent directions as is 

 the case with the tentacles. The lower part of this eye 

 stalk is bounded by unpigmented epithelium, but as the 

 ej'e is approached, the amount of pigment granules in the 

 epithelium cells increases until the whole cell is almost 

 tilled. The pigment is of a dark brown or blackish colour. 

 In sections it will be seen that the epithelium is composed 

 of columnar cells, w^hich are lowest at the base but become 

 taller as one approaches the cornea {Cor.), which is a 

 continuation of the same layer over the surface of the eye. 



The pigment granules are thickest at the bases of the 

 cells, the nuclei of which are generally situated in the 

 upper part. The pigmented cells are not equally 



G 



