BY THE PRESIDENT. 
Ill 
be of service to animals seeking or avoiding places illuminated by 
the direct rays of the sun. Thus the snail or slug may be guided 
to shelter. It has, however, been suggested that the beautiful eyes 
of the pecten or common scallop, which are situated on the mantle, 
and number from 80 to 120, are organs for the emission of the 
phosphorescence* which it sometimes displays, but I have no reason 
to think this to be the case, and believe them to be eyes of high 
structural development.! Now, the sense of sight can be of no 
service to the pecten in obtaining food, which is brought to it by 
the water. Nor is vision really required for its protection against 
its enemies; for although it can move with some agility by closing 
its half-opened valves and forcibly expelling the water, yet like 
the oyster it can shut its hard encasement on the approach of 
danger. It is possible that its eyes serve to warn it of the ebbing 
tide, by being affected by the increasing intensity of light. 
I propose for the moment to confine my remarks to simple eyes 
as possessed by insects and spiders. The general form may be 
described as a capsule, the internal portion of which is lined with 
pigment, while the outer surface is transparent and convex, and is 
called the cornea. It is generally of sufficient thickness to act as 
a lens; hence the term corneal lens. Behind the lens is a semi¬ 
liquid which fills the optic chamber, the hinder part of which is 
lined with the retina, and any image it receives is reversed as in 
the human eye. Such is the structure of the eyes of spiders, of 
many larvae of insects, and of fleas, which, unlike most insects, have 
in their imago state two simple instead of compound eyes. 
The sense of sight possessed by insects with such eyes is prob¬ 
ably exceedingly limited, and it is quite likely that many of them 
are dependent more upon the senses of touch and smell for obtaining 
their food than upon the sense of sight. This seems confirmed 
when we consider that the larvae of the dragon-fly, which can only 
obtain their food by employing specialised organs to capture other 
aquatic insects, have even in their earliest stages compound eyes, 
while the random leaps of the flea, which frequently only bring it 
back to the danger from which it had just escaped, tend to show 
the inferiority of the simple eye. A further advance in the struc¬ 
ture of the simple eye is seen, for instance, in the caddis-flies, and 
in some of the Copepoda. The corneal lens is of reduced thickness, 
and immediately behind it is another of different refractive power. 
So far as the ocelli are concerned, many entomologists hold that 
their function is confined to the delicate perception of the various 
* ‘Proc. Acad. Nat. Science Philadelphia,’ 1886, p. 61. 
f See “ The Eye of the Pecten,” ‘ Quarterly Journal Micro. Science,’ 1880. 
