out as they are extended? Is the respiratory opening on the right or left side of the body? On the mantle fold or on the body? (Figs. 202-3-4.) How often does the aperture open and close?
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Fig. 203.—A Slug.
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Fig. 204.—Circulation and Respiration in Snail.
a, mouth; b, b, foot; c. vent; d, d, lung; h, heart.
Blood vessels are black. (Perrier.)
Place the snail in a
moist tumbler. Does
the whole under surface seem to be used in creeping? Does the
creeping surface change shape as the snail creeps? Do any folds
or wrinkles seem to
move either toward the
front or rear of its
body? Is enough mucus
left to mark the
path traveled? The
fold moves to the front,
adheres, and smooths
out as the slug or snail
is pulled forward.
Cephalopods.—The highest and best developed mollusks are the cephalopods, or "head-footed" mollusks. Surrounding the mouth are eight or ten appendages which serve both as feet and as arms. These appendages have two rows of sucking disks by which the animal attaches itself to the sea bottom, or seizes fish or other prey with a firm grip. The commonest examples are the squid, with a long body and ten arms, and the octopus, or devilfish, with a short body and eight arms. Cephalopods have strong biting mouth parts and complex eyes somewhat resembling the eyes of backboned, or vertebrate, animals. The large and staring eyes add to the uncanny, terrifying appearance.
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Fig. 205.—A Squid.
The sepia or "ink" discharged through the siphon of the squid
makes a dark cloud in the water and favors its escape from
