SEA-FISHERIES LABORATORY. 429 
left upper valve are the more numerous, are larger and 
situated on longer stalks in P. maazmus. 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). Neither 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, LH. st.) which supports the 
eyes is a short column, and is retractile, but one does not 
observe the eyes being turned in different directions as is 
the case with the tentacles. The lower part of this eye 
stalk is bounded by unpigmented epithelium, but as the 
eye 1s approached, the amount of pigment granules in the 
epithelium cells increases until the whole cell is almost 
filled. ‘The pigment is of a dark brown or blackish colour. 
In sections it will be seen that the epithelium is composed 
of columnar cells, which 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 
EE 
