MOLJLUSCA. 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. Ixxxiv. 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 uvea and iris, is strengthened by a membrane 

 which appears to be muscular, and probably assists in the 

 contraction or enlargement of the aperture. 



