_Pearls_ are deposited by bivalves around some irritating particle that gets between the shell and the mantle. The pearl oyster furnishes most of the pearls; sometimes pearls of great value are obtained from fresh-water mussels in the United States. Name articles that are made partly or wholly of mother-of-pearl. [Illustration: FIG. 202.--A SNAIL. _l_, mouth; _vf_, _hf_, feelers; _e_, opening of egg duct; _fu_, foot; _ma_, mantle; _lu_, opening to lung; _a_, vent.] =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 has it? 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 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? [Illustration: FIG. 203.--A SLUG.] [Illustration: 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. [Illustration: 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 enemies almost as much as 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.