PRESENCE OF EYES IN SHELLS OF CERTAIN CHITONIDCE. 47 
exactly the same manner (PI. VI, fig. 6). One side the 
bulb of the pear, more or less near its extremity, is closely 
applied to the outer surface of the tegmentum (Pl. YI, 
figs. 4, 5), and here its wall is pierced by a circular aperture, 
the pupil-like opening. This opening is covered by the cornea, 
the periphery of which extends to a considerable distance 
beyond its margin all round (PI. YI, fig. 6,/). 
The cornea is a concavo-convex, watchglass-shaped lamina. 
It is calcareous in structure, being continuous all round its 
margin with the superficial calcareous layer of the tegmentum. 
It resists the action of strong boiling caustic alkalies, but col- 
lapses at once when treated with acid. In sections of the 
uudecalcified tegmentum it shows itself to be formed of a 
series of concentric lamellae of transparent hard substance. 
Probably a continuation of the cuticular substance of the teg- 
mentum is present in its substance, but I have been unable to 
demonstrate the existence of such by means of acids. 
The pear-shaped cavity of the eye in the tegmentum is lined 
by a dark brown pigmented membrane, of a stiff and apparently 
somewhat chitinous texture, which forms the eye capsule. 
This capsular membrane exactly follows the shape of the eye 
cavity, except near the surface of the tegmentum, where its 
margin curves inwards beneath the cornea, forming a sort of 
iris and bounding the circular pupil, which, as before men- 
tioned, is of less diameter than the cornea. The aperture of 
the pupil is occupied by the front surface of the lens. The 
lens is perfectly transparent and hyaline and strongly biconvex, 
it is filled in behind the iris aperture. It is composed of soft 
tissue and dissolves in strong acetic acid gradually and com- 
pletely, showing a fibrous distinct structure in the process. 
In Acanthopleura spiniger the lens is a little flatter in 
front than behind (PI. YI, fig. 6, g). There is a space between 
the front surface of the lens and the cornea. 
The optic nerve at some distance from the eye, where 
arising from the general ramification, is a compact strand 
completely identical in structure in Acanthopleura, with 
the strands proceeding to the megalsesthetes. In Onithochiton 
