NOTES ON THE GENUS MONSTRILIA. 569 
The eyes have been variously described by different 
authors ; Dana and Claparéde describe two simple eyes above, 
and a single median eye on the lower side. Semper describes 
a single non-facetted eye, consisting apparently of a single 
hemispherical lens, surrounded by pigment. Claus describes 
a single upper eye with two large lenses, and Thompson has 
given a similar account of the eyes of his Cymbasoma 
rigidum. Claparéde states that the males of M. Dane are 
entirely devoid of eyes. 
My own observations confirm those of Dana and Claparéde. 
In the males of M. anglica and the females of M. rigida 
one sees on the upper surface of the anterior part of the head 
an apparently single median eye, consisting of two lenses 
embedded in cup-shaped masses of dark brown pigment. The 
lenses are placed back to back so as to look laterally and 
somewhat anteriorly. On the ventral surface of the head is a 
third median lens embedded in pigment which is continuous 
with that of the upper lenses. The specimens of which I have 
been able to cut sections were not well enough preserved for 
me to be able to give account of the minute structure of these 
eyes, but from what I can make out they must be described as 
three monomeniscous eyes placed back to back. The eyes 
appear to differ in some other species. I have, for instance, 
an apparently eyeless male, similar to that figured by Claparéde, 
which in all other features resembles M. anglica, and 
in M. longispinosa the median lower eye appears to be 
absent. 
Situated at a short distance behind the antennz, on the 
ventral surface of the head, are a pair of pits, which may be 
seen in fig. 1, and one of them is represented highly magni- 
fied in fig. 4. The exact nature of these pits is not yet 
clear to me. At first I was inclined to consider them as the 
rudiments of the second antenne, but a further study leads 
me to believe that they are the openings of glands. In some 
specimens the aperture at the base of the basin-shaped depres- 
sion of the cuticle appears to be surrounded with glandular 
cells, and in fig. 5 two masses of cells embedded in pigment 
