MOLLUSCA.- 123 
classes of animals, with the coloured mucous pigment which 
has been denominated pigmentum nigrum. In the cepha- 
lopoda, however, it is of a purplish-red colour. 
The optic nerve, after entering the sclerotica, expands 
into a large ganglion, from the peripheral surface of which, 
issue numerous nervous filaments. These pierce the cho- 
roides by as many holes, and go to form, by their reunion, 
the retina. This important membrane extends to the cili- 
ary zone, and, like it, appears to unite itself with the groove 
of the lens. 
The vitreous humour is contained in a peculiar vesicle, 
having the lens seated in a concavity on its external sur- 
face. The lens divides easily into two parts, the line of 
separation being the groove which receives the ciliary liga- 
ment. The separated surfaces are flat, and the outer por- 
tion is in the form of a planoconvex lens. Each portion 
consists of a number of concentric layers of variable thick- 
ness, composed of radiated fibres, becoming less and less 
distinct towards the centre, near which the laminated and 
radiated appearances cease to be perceptible. An imper- 
fect representation of this structure is given by Sir E. Home, 
probably from preparations: by Mr. John Hunter, in the 
Phil. Trans. vol. |xxxiv. tab. 5. p. 26. 
The conjunctiva supplies the place of a cornea, and covers 
directly the crystalline lens, as there is no aqueous humour. 
This membrane, in some, is continuous with the skin, but 
in others, there are imperfect eye-lids formed by its dupli- 
_ cature, previous to passing over the lens. The skin, at the 
opening of the pupil, formed by the sclerotica, in the ab- 
sence of an wvea and iris, is strengthened by a membrane 
which appears to be muscular, and probably assists in the 
contraction or enlargement of the aperture. 
