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Conclusion. 
I have already referred to the difference which exists between 
Vertebrata and Invertebrata as to the arrangement of the optic 
nerve upon the retina. The eyes of the tentacles of the Onchidia 
(slug-like Molluscs) are not exceptional in this respect, but the 
greater number of the species of this genus have on their shell-less 
hacks other eyes, the optic nerve of which enters in the same way 
as with the Yertebrata. In some individuals there are as many as 
one hundred eyes. Such Onchidia live in tropical regions on the 
sea-shore, or in brackish marshes, and obtain their nourishment 
from the organic particles mixed with the sand as they creep along 
close to the edge of the water. They thus fall an easy prey to 
certain fishes (of the genera Periophthalmus and Boliophthalmus) 
which skip along the water-line and even on the land where they 
can live for some time. Now the hack of the Onchidium is studded 
with glands with minute orifices capable of expelling a thick 
secretion. It is supposed that the Mollusc is warned of danger 
by the shadow of the leaping fish falling on its dorsal eyes. It 
then contracts its body and emits with considerable force its 
secretion, which if not offensive or injurious may at least disturb 
the fish and divert its course. Semper* has observed the habits of 
the Onchidia, and he points out that, so far as his experience goes, 
those species with dorsal eyes have precisely the same distribution 
as the two fishes already named, whereas where the fishes are not, 
there the Onchidia have no dorsal eyes. In our English species 
( 0 . celticum ), as indeed in many others, these highly-developed 
glands and dorsal eyes are both absent. This is as we should 
expect, for the two sets of organs seem so closely related for 
defensive purposes that one is next- to useless without the other. 
Time prevents the consideration of the interesting question as to 
the development of these dorsal eyes, or as to how far in such 
species as our own, these organs, together with the associated 
weapons of defence, may at one time have existed, and have 
become abortive through disuse. It is sufficient for our present 
purpose to point out the apparently remarkable adaptation of the 
dorsal eyes to the wants of the organism possessing them. 
The degeneration of the eye from disuse, followed as it is 
frequently by blindness, is familiar to most of us. Thus people who 
squint and who saw at first all objects double, may lose in time 
the sight of one eye, by constantly suppressing its image in order 
* See “ Ueber Schnecken-Augen v. Wirbelthiertypus,” by Semper, ‘Arckiv 
f. Mikroskopiscbe Anatomie,’ Band xiv, p. 118 ; also by the same author, ‘ Animal 
Life,’ pp. 281 and 378, and ‘ Eeisen im Archipel der Philippine^’ Wiesbaden, 
1877, 2nd part, vol. iii. 
