74 Miscellaneous. 
duced to explain the phenomenon of vision. As the author points 
out, the case of albinos shows that luminous perceptions do not 
cease when the pigment is absent; and we do not know any ease in 
which the pigment is absolutely necessary for the perception of light. 
The pigment really fulfils two functions. In the first place it 
absorbs the superfluous light and prevents it from being reflected 
upon other parts of the retina; and it arrests all the luminous rays 
which may penetrate into the eye by any other road but the pupil, 
whether through the cornea or the sclerotic. This second function 
is of particularly great importance in the Mollusca. In many of 
these (for example, in the Heteropoda) the parts surrounding the 
eye are perfectly transparent; and even in the snails the ommato- 
phore is sufficiently transparent to allow us sometimes to see the 
outer layer of the retina without any difficulty. The luminous rays 
may, therefore, strike this outer layer of the retina in all directions ; 
and from this M. Hensen justly concludes that it cannot be sensitive 
to light. Sensibility to luminous rays consequently appears to be 
peculiar to the inner layer, accessible only to the rays which 
have passed through the crystalline. This layer alone is com- 
parable to the stratum of bacilli in the Vertebrata. 
In comparing the eyes of the Mollusca with those of other 
animals, M. Hensen directs attention to the difficulty resulting from 
the variable meanings of terms. The words eye, retina, iris have 
acquired a perfectly definite physiological sense; but this is not the 
case with the words sclerotic, cornea, and choroid, because they are 
used to designate organs with various and still ill-defined functions. 
Thus, for example, the sclerotic serves at once as the protective en- 
velope of the eye, as the support of the cornea, and as the basis for 
the attachment of muscles, without its being possible to say that 
any one of these functions is more essential than the rest. In the 
Vertebrata the sclerotic and the neurilemma of the optic nerve are 
justly regarded as prolongations of the dura mater. This morpho- 
logical character ought, apparently, to be the best guide in the in- 
vestigation of the homologies of the sclerotic; but when we come 
to animals in which the eyes are not formed as if by a budding of 
the brain, and in which we can find no dura mater, it becomes very 
difficult any longer to speak of a sclerotic. 
With regard to the structure of the retina there is, between the 
Vertebrata and the Invertebrata, a chasm which seems to defy all 
homologies. Thus in the former the bacilli form the outer layer 
of the retina; in the latter they form its inner layer. And yet it 
is remarkable that, notwithstanding this fundamental difference in 
the typical organization of the essential part of the organ of sight, 
the uniformity of organization persists in the accessory organs. 
Thus, as M. Hensen remarks, in the Cephalopoda, the crystalline 
continues to be an epithelial production, resulting from an invagi- 
nation of the skin, as in the Vertebrata. In these higher Mollusca 
there also exist an iris, a cornea, and eyelids—organs which, it is 
true, disappear one after the other in this class of animals. The 
sclerotic appears as if divided into fragments. One portion forms the 
