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tatives of these different types have eyes of very considerable 
complexity and of very different ancestry ; and all exhibit obe- 
dience to the same physical laws, and present similar optical 
conditions. 
Simpler than these are the eyes of the Star-fishes, and still 
simpler those of the Leech and of the lowlier Worms, some of 
which have, as it seems, nothing more than mere pigmented 
spots, the nervous supply of which has still to be worked out. 
But the diversity does not end here : the Leech has ten 
< eyes.’ Some of the marine Worms that live in tubes have a 
number of eyes on the delicate respiratory filaments which pro- 
ject from their anterior end ; some have eyes at the hinder end 
of their body ; and Polyophtha Imus is a veritable Argus, for [in 
addition to the eyes in its head there is a pair on every segment 
of the body. 
Where so much variation is seen it is only natural to expect 
that external conditions will be found to have very consider- 
able influence. We all know that the Earth-worm is without 
eyes ; and to the various known examples of the reduction of 
the eyes in such burrowing animals as the mole, and the com- 
plete blindness of some that live in caves, such as the blind- 
fish (Ambit) opsis) of the Mammoth Cave of Kentucky, and the 
amphibian Proteus of the caves of Carniolia, there have lately 
been added two blind snails from the just-named Austrian 
grotto. 
The great increase in our knowledge of animals which has 
resulted and is resulting from the deep-sea explorations which 
have been carried on with such activity during the last few 
years, is very strikingly shown in the case of the organ of 
sight. In one of the earliest of these : — that of the Porcupine 
— a crustacean (Ethusa granulata ), was discovered living at 
various depths; specimens taken in shallow water were found to 
have eyes of the usual character ; those which were brought up 
from depths varying between 110 and 370 fathoms had the 
ordinary eye- stalks, but the place of the eye was taken by a 
calcareous knob ; the condition was one to which the words of 
Mr. Darwin may well be applied, ‘ The stand for the telescope 
is there, though the telescope with its glasses has been lost/ 
Specimens of the same species taken from still greater depths 
(from 500-700 fathoms) exhibited a still more remarkable 
change : the eye- stalk was found to have lost its special cha- 
racter, and to have been converted into a pointed rostrum. So 
again a number of new blind Crustacea have been lately 
described by M. Milne-Edwards from the Gulf of Mexico. 
Did the matter end here it would be easy enough to ascribe 
the changes to the disuse of the eye, owing to the want of 
light at so great a depth. Fortunately, however, naturalists 
