46 
PROFESSOR H. N. MOSELEY. 
On the intermediate or middle shells the eyes are confined 
to the arese laterales or to the lines of demarcation between 
the areae laterales and the area ventralis, which latter is usually 
entirely devoid of them. The eyes, which are mostly circular 
in outline as seen on the shell surfaces, measure about T . 3 of an 
inch in diameter, in Schizochiton incisus of an inch, in 
Acanthopleura spiniger and in Corephium aculeatum, 
in which they are oval in outline, -,-L- of an inch by about 
In Enoplochiton they are smaller still, and only with 
difficulty seen at all. The eyes appear when viewed by 
reflected light with a simple lens or low power of the compound 
microscope as highly refracting convex circular spots, looking 
as if made of glass or crystal (see PL IV, figs. 1, 2, 3, 4). 
The highly refracting spot, the cornea, is set off by a surrounding 
narrow zone of dark pigment, which is the margin of the pig- 
mented eye capsule which forms an iris-like structure round 
the lens, and which is seen through the superficial shell 
substance (PI. IV, fig. 3). Through the centre of each cornea 
is seen a smaller circular area, somewhat darker than the 
aperture of the pupil, but showing a brilliant spot of totally 
reflected light due to the lens. 
Structure of the Eyes. 
The eyes are evidently to be regarded as having arisen as 
modifications of megalaesthetes. They are connected with the 
same network of soft tissues as terminal organs of its ramifica- 
tions in the same manner, and have points of resemblance to 
them which are convincing as to the homogeny of the two. 
The soft structures of each eye lie in a more or less pear- 
shaped chamber, excavated in the substance of the tegmentum. 
The stalk of the pear, which forms the canal for the passage of 
the optic nerve, is directed always towards the free margin of 
the tegmentum whence the nerve reaches it. In Acantho- 
pleura the eye chambers and the neural canals continued from 
them follow in direction the same course as the megalopores 
and their canals, and join the main canal ramifications in 
