THE NERVOUS SYSTEM. 
181 
Fig. 158. — Eye of 
Pecten, much en¬ 
larged: m, mouth; 
l, lens; r, retina 
and choroid; w, 
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 oroid) 
and a ner¬ 
vous net¬ 
work (reti¬ 
na) behind. 
The Scallop 
{Pecten) has 
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, 
green, purple, brown, or black. Under 
the microscope, the surface is seen to 
be divided into a host of facets, 95 each 
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; Jc , nerve and muscle of left 
inferior tentacle; Z, m, nervous collar. 
Fig. 155.—Head of the Bee, 
showingcompoundeyes, 
the three ocelli, or stem- 
mata, and the antennae. 
Magnified. 
being an ocellus 
complete in itself. Each cornea is convex on one side, 
