T11E NERVOUS SYSTEM. 
181 
Fig. 15a. — Eye of 
Pecten, much en¬ 
larged: m, mouth; 
l , lens; r, retina 
and choroid ; n , 
nerve. 
Going higher, we find a lens introduced forming a dis¬ 
tinct image. The Snail, for example, has two simple eyes, 
called ocelli , mounted on the tip of its long tentacles, con¬ 
sisting of a globular lens, 94 
with a transparent skin 
(cornea) in front, and a 
colored 
membrane 
(ch o r o i d) 
and a ner¬ 
vous n e t- 
work (reti¬ 
na) behind. 
The Scallop 
(. Pecten ) lias 
such eyes in the edge of 
its mantle (Fig. 153). Such 
organs are the only eyes 
possessed by Myriapods, 
Spiders, Scorpions, and 
Caterpillars. Adult In¬ 
sects usually have three ocelli on the top of the head. 
But the proper visual organs of Lobsters, Crabs, and In¬ 
sects are two compound eyes , perched 
on pedestals, or fixed on the sides of 
the head. They consist of an immense 
number of ocelli pressed together so 
that they take an angular form—four¬ 
sided in Crustacea, six-sided in Insects. 
They form two rounded protuberances 
variously colored — white, yellow, red, Fig. 155—Head of the Bee, 
green, purple, brown, or black. U rider the three ocein, or stem- 
the microscope, the surface is seen to M^umed.^ 6 auteunae ' 
be divided into a host of facets, 95 each being an ocellus 
complete in itself. Each cornea is convex on one side, 
Fig. 154. —Head of a Snail bisected, showing 
structure of tentacles: a, right inferior ten¬ 
tacle retracted within the body; b, right su¬ 
perior tentacle fully protruded; c, left supe¬ 
rior tentacle partially inverted ; d, left inferi¬ 
or tentacle;/, optic nerve ; g, retractor mus¬ 
cle; h, optic nerve in loose folds; i, retractor 
muscle of head ; k, nerve and muscle of left 
inferior tentacle ; l, m, nervous collar. 
