206 Animals with more than Two Eyes. 
the eye like our own; a lens for forming the picture of outside 
objects, an optic nerve, and other accessories for the purposes of 
vision. 
Very remarkable in so humble a creature is the protection of 
the lower sides of the eye-ball with a dark coloured pigment, 
which prevents the access of too much side light. The micro- 
scope tells us much more about these eyes of the scallop. 
Another animal endowed with more than two eyes is found 
amongst the various creatures known as Onchidia. These 
animals, which are sea-slugs, live exclusively on the sea-shore or 
in brackish marshes. They are found in the Philippine Islands, 
and in certain parts of the southern coast of Australia. For our 
knowledge of their structure and the strange position in which 
their extra eyes are placed, we are chiefly indebted to Herr Carl 
Semper, professor in the University of Wiirzburg. 
Onchidium, like other slugs, has two eyes on its head, in the 
usual place; but it also possesses a large number of eyes on its 
tough, leathery back! These dorsal eyes, as they are called, 
have been found in more than twenty species of Onchidia. 
Professor Semper has counted as many as ninety-eight on the 
back of a single Onchidium. 
These eyes on the back of the animal occur in groups in 
some species, and singly in other species. The younger speci- 
mens have the greatest number. When the skin of the animal 
is rough, and raised into little hills, the eye or eyes will be found 
at the summit. In these cases the eye is retractible ; that is, it 
can be drawn in so as to avoid the dangers to which its elevated 
position exposes it. 
The Onchidium, then, is better off than the scallop, inasmuch 
as it has a head, and a multiplicity of eyes in addition. But why 
should it have eyes on its back? Such eyes are chiefly directed 
upwards to the sky, and are quite useless for looking down on the 
earth, where the food of the animal lies. But it is fairly certain 
that these dorsal eyes are no purposeless “freak of nature.” 
There is very good reason to believe that they serve to warn the 
animal of the attacks of a fish which seeks to prey upon it above, 
leaping upon it through the air. 
But some shell-fish greatly excel the Onchidia in the number 
of their eyes. The so-called coat-of-mail shells, or Chitonidze, 
are perhaps the most marvellous myriad-eyed animals we know of. 
Some of them have as many as eleven thousand eyes. We may 
well smile at the comparative poverty of the mythological Argus 
in the matter of eyes when we look at one of these coat-of-mail 
shells. But the strangest thing about these thousand-eyed 
animals is yet to be told. Their eyes are not found on the body, 
as in the case of the scallop; you will look in vain for them upon 
