. Beginners' Zoology . to the mouth. As the water passesalong the perforated gills, its oxygenis absorbed ; the mantle also absorbsoxygen from the water as it water, as stated before, goesnext through a passage between thefoot and the palp into the cavity a-bove the gills and on out throughthe exhalent siphon. By stirringthe water, or placing a drop of inknear the siphons of a mussel keptin a tub, the direction of its flowmay be seen. The pulsations ofthe heart are plainly visible in aliving mollusc. Habits of the it abundant in clear or in mud-dy w^ater; swift, still, or
. Beginners' Zoology . to the mouth. As the water passesalong the perforated gills, its oxygenis absorbed ; the mantle also absorbsoxygen from the water as it water, as stated before, goesnext through a passage between thefoot and the palp into the cavity a-bove the gills and on out throughthe exhalent siphon. By stirringthe water, or placing a drop of inknear the siphons of a mussel keptin a tub, the direction of its flowmay be seen. The pulsations ofthe heart are plainly visible in aliving mollusc. Habits of the it abundant in clear or in mud-dy w^ater; swift, still, or slightly moving water? Describeits track or furrow. What is its rate of travel. Can youdistinguish the spots where the foot was attached to theground .-* How long is one step compared to the lengthof the shell. The animal usually has the valves openedthat it may breathe and eat. The hinge ligament acts likethe case spring of a watch, and holds the valves open un-less the adductor muscles draw them together (Fig. 189).. Fig. 198. — Diagram ofMussel cut across,showing mantle, ma; gills,kie; foot, /; heart, h; in-testine, ed. I04 BEGINNERS ZOOLOGY When the mussel first hatches from the egg, it has a tri-angular shell. It soon attaches itself to some fish and thustravels about. After two months itdrops to the bottom again. Other Mollusca. — The oyster s shellsare not an exact pair, the shell whichlies upon the bottom being hollowedout to contain the body, and the uppershell being flat. Can you tell by ex-amining an oyster shell which was thelower valve ? Does it show signs ofhaving been attached to the bottom ?The young oyster, like the young mus-sel, is free-swimming. Like the arthropoda, most molluscsundergo a metamorphosis to reachthe adult stage (Fig. 199). Examine the shells of clams,snails, scallops, and cockles. Make
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