Study of a Live Snail or Slug.—Is its body dry or moist? Do land snails and slugs have lungs or gills? Why? How many pairs of tentacles have they? What is their relative length and position? The eyes are dark spots at bases of tentacles of snail and at the tips of the rear tentacles of slug. Touch the tentacles. What happens? Do the tentacles simply stretch, or do they turn inside out as they are extended? Is the respiratory opening on the right or the 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?
Fig. 203.—A Slug.
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 the rear of its body? Is enough mucus left to mark the path travelled? The fold moves to the front, adheres, and smooths out as the slug or snail is pulled forward.
Fig. 204.—Circulation and Respiration in Snail.
a, mouth; b, b, foot; c, vent; d, d, lung; h, heart. Blood vessels are black. (Perrier.)
Cephalopods.—The highest and best developed molluscs are the cephalopods, or “head-footed” molluscs. 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.
Fig. 205.—A Squid.
The sepia or “ink” discharged through the siphon of the squid makes a dark cloud in the water and favours its escape from enemies almost as much as does its swiftness (Fig. [205]). The squid sometimes approaches a fish with motion so slow as to be imperceptible, and then suddenly seizes it, and quickly kills it by biting it on the back behind the head.