PRESENCE OE EYES IN SHELLS OE OERTAIN OHITONIDJ;. 13 
exactly thè same manner (PI. VI, fig. 6). One side thè 
bulb of thè pear, more or ìess near its extremity, is closely 
applied to thè outer surface of thè tegmentum (PI. VI, 
figs. 4, 5), and bere its wall is pierced by a circular aperture, 
thè pupil-like opening. This opening is covered by thè cornea, 
thè periphery of which extends to a considerable distance 
beyond its margin all round (Pi. VI, fig. 6,/). 
The cornea is a concavo-convex, watchglass-shaped lamina. 
It is calcareous in structure, being continuous all round its 
margin with thè superficial calcareous layer of thè tegmentum. 
It resists thè action of strong boiling caustic alkalies, but col- 
lapses at once when treated with acid. In sections of thè 
undecalcified tegmentum it shows itself to be formed of a 
series of concentric lamellse of transparent hard substance. 
Probably a continuation of thè cuticular substance of thè teg- 
mentum is present in its substance, but I bave been unable to 
demonstrate thè existence of such by means of acids. 
The pear-shaped cavity of thè eye in thè tegmentum is lined 
by a dark brown pigmented membrane, of a stififand apparently 
somewhat chitinous texture, which forms thè eye capsule. 
This capsular membrane exactly follows thè shape of thè eye 
cavity, except near thè surface of thè tegmentum, where its 
margin curves inwards beneath thè cornea, forming a sort of 
iris and bounding thè circular pupil, which, as before men- 
tioned, is of less diameter than thè cornea. The aperture of 
thè pupil is occupied by thè front surface of thè lens. The 
lens is perfectly transparent and hyaline and strongly biconvex, 
it is filled in behind thè iris aperture. It is composed of soft 
tissue and dissolves in strong acetic acid gradually and com- 
pletely, showing a fibrous distinct structure in thè process. 
In Acanthopleura spiniger thè lens is a little flatter in 
front than behind (PI. VI, fig. 6, g). There is a space between 
thè front surface of thè lens and thè cornea. 
The optic nerve at some distance from thè eye, where 
arising from thè generai ramification, is a compact strand 
completely identical in structure in Acanthopleura, with 
thè strands proceeding tothe megalaesthetes. In Onithochiton 
