aresoftandthey do nothave internalskeletonsorbackbones,they are called
invertebrateanimals.
Naturallytheseanimalsneedsomemeansofprotection,particularlysince
theirbodiesare soft. Toprovidethis,theirbodycellssecretethehardprotec-
tivesubstanceswe call shells. Crabsandcrayfishshed theiroutgrownshells
as theirbodiesgrowlargerand producenewandlargershells to fit asneeded-
On theotherhand,clams,oysters,musselsandsnailsdo notleavetheirshells
as they grow. Theirbodiessecretenew cells toaddto theoriginalonesand
moreshellmaterialtoaddto theirsize. Whenacrabhasjustformeda new
shell thathasnotyethardened, it iscalleda "soft-shelledcrab." Thisterm
designatesa stage in thedevelopmentofa crab.
All overthe world the sea animalsyouhave collectedare valuedas a
sourceoffood.
STUDYING ASTARFISH
Follow thisprocedure: Examine the dried starfishInyourcollection of
shellsandseaanimals.
You will observe:Thereis aroundopeningin themiddleoftheunderside
ofthestarfish. Alongthe undersurfaceofeach"arm"oftheanimalthereare
tworowsoflittlestem-likesuctioncups.Thecentralopeningintheundersideofthe starfishacts as amouththat
pullsfooddirectlyintothe sac-likestomachofthe animal.
Thesuctioncupsalongitsarmshelp the animalstick to thehardsurface
oflargeranimalsand torocksas it movesitsmusculararmsinlocomotion.
An oysterprovidesa deliciousmeal for a wilystarfish. Andthestarfish's
meansofcatchinganddevouringhispreyisquitefascinating.
Thestarfishslowlymovesup on theunsuspectingoysterwhomaybejust
relaxingatthe bottomofa shallowpartofthesea. Thestarfish crawlsover
his futuremeal and wrapshimselffirmly around the two-partshell ofthe