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1、clickforpreviouspageGASTROPODSbyJ.H.Leal,TheBailey-MatthewsShellMuseum100GastropodsGENERALREMARKSheGastropodaconstituteoneofthemostspecioseanddiversegroupsofanimals,comprisingmorethanThalfofallnamedmolluscs(attheclassrank,GastropodaissecondonlytotheInsectainnumberofspe-cies).Thewidelyvariedarrayofb
2、odyplansandshellshapesintheGastropodareflectsthediversepathsonwhichthegroupradiatedsincetheCambrian.Asbrieflydiscussedbelow,thisdiversityisaresultofsomebasicconstraintsdefinedearlyintheevolutionaryhistoryoftheclass.Gastropodsareasymmetricalmolluscsthatunderwenttorsion.Thebodyisgenerallydividedinto2
3、mainre-gions:(1)head-footand(2)mantle(includingshell),mantlecavity,andvisceralmass.Inmostgastropodsthemuscularfootisthelocomotionorgan;gastropodsmainlycrawl,attach,orburrowusingthefoot.Theheadin-cludessenseorgans(e.g.,tentaclesandeyes)andinmanygroupsisthesiteofconcentrationofnervegangliaandconnecti
4、ves.Themantle,typicalofmolluscs,linestheshellinternally;itsexternaledgeisthesiteofshelldeposition.Thespacebetweenthehead-footandthemantleproperisthemantlecavity,wherethectenidium(orctenidia),osphradium,anus,nephridiopore,andexternalgenitaliaarelocated.Thevisceralmass,locatedinposteriordirection,ist
5、helocationofthegonads,digestivegland,heart,kidney,andpartofthealimentarysystem.TheGastropodaarealmostuniversallyacceptedasamonophyleticgroup.Gastropodsaredefinedbythepresenceofcharacterssuchasalarvaloperculum,butmainlybyundergoingtorsionanddisplayingassoci-atedanatomicalconditions,suchasanincreased
6、concentrationoforgansinthevisceralmass.Torsion(nottobeconfusedwithcoilingoftheshellaroundapointoranaxis)istherotationofthevisceralmassandmantle(includingshell)forupto180ºinrelationtotheheadandfoot,alwaysinacounterclockwisedirection,andmostfrequentlyinthelate-veligerlarvalstage.Duringtorsion,themant
7、lecavityanditsorgansrotatefromaposte-riortoananteriorposition.Asaresult,thelongitudinalnervecordsconnectingthevisceraltothepleuralgan-gliainafullytortedgastropodformafigure-eight,andthedigestivetractistwist
