264 i DR. A. RATTRAY, ON THE ANATOMY, PHYSIOLOGY, 
niferous Gasteropods. Like minute black specks to the unaided eye (Plate XLIV. figs. 13 
&c.), they are hour-glass-shaped under the microscope, and readily distinguished by the 
crimson hue of the retina, which is only equalled in brilliancy by the eyes of some 
oceanie Myriopoda occasionally found in the towing-net. The component parts are seven 
in number, viz. :— 
a. The optie nerve (Plate XLIII. fig. 8, 2), which has already been described. 
b. The sclerotic (fig. 8, ab) is cup-shaped, its cavity looking outward and forward, 
moderately thick, becoming thinner and narrowing in calibre in front, and constricted 
about one-third from its anterior orifice, where a sharp ridge (a d) projects into its cavity, 
and divides it into a small anterior and a larger posterior chamber; and in front joined 
by the thin posterior edge (« e) of the external eye-cover or cornea that likewise con- 
 tributes slightly to form the anterior chamber occupied by the vitreous humour. 
c. The retina (ac). The interior of both chambers, semigranular and bright crimson, 
is lined by the expansion of the optic nerve, which penetrates the thick posterior part of 
the sclerotic ; but the manner in which this is effected, and the minute structure of the 
retina, need further inquiry. The colour may be the natural tint of the retina ; but as 
the nervous system generally, both ganglia and cords, and the optic nerve itself, close to 
where it enters the eye, have only the usual pale yellow tinge previously spoken of, we 
may consider it due to the presence of the following or 
d. Pigmentary layer; but whether this exists beneath the retina, as in the human 
subject, or overlies it, as in the Cephalopoda (Owen), it is not easy to determine. 
e. The vitreous humour. The contents of the sclerotic cavity, which correspond to 
the vitreous humour of the human eye, are hyaline, and apparently structureless. 
f. The crystalline lens (a f), globular, pellucid, and highly refractive, is situated in front 
of the vitreous humour, about two-thirds of its anterior being covered by the cornea ; while 
its posterior third adjoins the vitreous humour, and makes a segmental projection into the 
sclerotic. When subjected to pressure after immersion in spirit, the lens splits towards . 
its centre, like those of higher animals. 
9. The cornea (ag) is merely a prolongation of the general outer envelope, like 
which it is perfectly transparent and structureless. Thick in front, it gradually thins 
as it bends back to embrace the crystalline lens and join the anterior edge of the 
sclerotic. This pellucid concavo-convex eye-cover, with the globular crystalline body that _ 
closely fits into it, and the vitreous humour behind all, constitute an admirable complex 
and powerful lens, doubtless well adapted for the requirements of sight. The constriction 
by which the cavity of the sclerotic and the contained vitreous humour are divided 
into two (ad), evidently performs the function of a rudimentary pupil, perhaps quite 
fixed, but possibly acted on in some measure by those muscular fibres so peculiarly : 
arranged around the eyes, apparently in connexion with vision. Posteriorly, and on the 
sides, the eyes are surrounded by the general tissues, and are not contained in any space 
like the orbital cavity of the Vertebrata, and have no special muscles, like the iris in the 
interior of, or the recti and oblique around the human organ. But the structure now 
alluded to suggests that, by the single or conjoint action of the longitudinal and trans- 
verse fibres, the direction of the axis of each eye may be materially altered at will, so as 
