178 COMPARATIVE ZOOLOGY. 
tion of light is the sensation of warmth, the pigment ab- 
sorbing the rays and converting them into heat. 
Going higher, we find a lens introduced forming a dis- 
AA il imnstlian 
Fig. 150.—Head of a Snail bisected, showing 
structure of tentacles: a, right inferior ten- 
tacle retracted within the body; 6, right su- 
perior tentacle fully protruded ; ec, left supe- 
rior tentacle partially inverted ; d, left inferi- 
or tentacle ; f, optic nerve; g, retractor mus- 
cle; h, optic nerve in loose folds; 7, retractor 
muscle of head; &, nerve and muscle of left 
inferior tentacle ; J, m, nervous collar. 
tinct image. The Snail, 
for example, has two sim- 
ple eyes, called ocedia, 
mounted on the tip of its 
long tentacles, consisting 
of a globular lens,’ with 
a transparent skin (cor- 
nea) in front, and a col- 
ored membrane (choroid) 
and a nervous net-work 
(retina) behind. 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 Insects 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 col- 
ored—white, yellow, red, green, pur- 
ple, brown, or black. Under the mi- 
croscope, the surface is seen to be di- 
vided into a host of facets,"* each be- 
Fig. 151.—Head of the Bee, 
showing compound eyes, 
the three ocelli, or stem- 
mata, and the antenne. 
ing an ocellus complete in itself. Each cornea is convex 
on one side, and either convex or flat on the other, so that 
