IntheCalamariesandthePoulpstheink-bagisplacedintheupperpartofthebody，incloseproximitytothemytis，whereasintheSepiaitislowerdown，againstthestomach。FortheSepiahasamoreplentifulsupplyofinkthantherest，inasmuchasitmakesmoreuseofit。Thereasonsforthisare，firstly，thatitlivesneartheshore，and，secondly，thatithasnoothermeansofprotection；whereasthePoulphasitslongtwiningfeettouseinitsdefence，andis，moreover，endowedwiththepowerofchangingcolour。Thischangingofcolour，likethedischargeofink，occursastheresultoffright。AstotheCalamary，itlivesfaroutatsea，beingtheonlyoneoftheCephalopodathatdoesso；andthisgivesitprotection。ThesethenarethereasonswhytheinkismoreabundantintheSepiathanintheCalamary，andthisgreaterabundanceexplainsthelowerposition；foritallowstheinktobeejectedwitheaseevenfromadistance。Theinkitselfisofanearthycharacter，inthisresemblingthewhitedepositonthesurfaceofabird’sexcrementandtheexplanationinbothcasesisthesame，namely，theabsenceofaurinarybladder。For，indefaultofthis，itistheinkthatservesfortheexcretionoftheearthiestmatter。
AndthisismoreespeciallythecaseintheSepia，becausethereisagreaterproportionofearthinitscompositionthaninthatoftheotherCephalopoda。Theearthycharacterofitsboneisaclearindicationofthis。ForinthePoulpthereisnoboneatall，andintheCalamaryitisthinandcartilaginous。WhythisboneshouldbepresentinsomeCephalopoda，andwantinginothers，andhowitscharactervariesinthosethathaveit，hasnowbeensetforth。
Theseanimals，havingnoblood，areinconsequencecoldandofatimidcharacter。Now，insomeanimals，fearcausesadisturbanceofthebowels，and，inothers，aflowofurinefromthebladder。
Similarlyintheseitproducesadischargeofink，and，thoughtheejectionofthisinkinfright，likethatoftheurine，istheresultofnecessity，and，thoughitisofexcrementalcharacter，yetitisusedbynatureforapurpose，namely，theprotectionandsafetyoftheanimalthatexcretesit。
TheCrustaceaalso，boththeCaraboidformsandtheCrabs，areprovidedwithteeth，namelytheirtwoanteriorteeth；andbetweenthesetheyalsopresentthetongue-likepieceofflesh，ashasindeedbeenalreadymentioned。Directlyaftertheirmouthcomesagullet，which，ifwecomparerelativesizes，isbutsmallinproportiontothebody：andthenastomach，whichintheCarabiandsomeoftheCrabsisfurnishedwithasecondsetofteeth，theanteriorteethbeinginsufficientforadequatemastication。Fromthestomachauniformgutrunsinadirectlinetotheexcrementalvent。
ThepartsdescribedaretobefoundalsoinallthevariousTestacea。Thedegreeofdistinctness，however，withwhichtheyareformedvariesinthedifferentkinds，andthelargerthesizeoftheanimalthemoreeasilydistinguishableareallthesepartsseverally。IntheSea-snails，forexample，wefindteeth，hardandsharp，asbeforementioned，andbetweenthemtheflesh-likesubstance，justasintheCrustaceaandCephalopoda，andagaintheproboscis，which，ashasbeenstated，issomethingbetweenastingandatongue。Directlyafterthemouthcomesakindofbird-likecrop，thenagullet，succeededbyastomach，inwhichisthemecon，asitisstyled；andcontinuouswiththismeconisanintestine，startingdirectlyfromit。ItisthisresidualsubstancewhichappearsinalltheTestaceatoformthemostpalatablemorsel。PurpurasandWhelks，andallotherTestaceathathaveturbinateshells，instructureresembletheSea-snail。ThegeneraandspeciesofTestaceaareverynumerous。Fortherearethosewithturbinateshells，ofwhichsomehavejustbeenmentioned；and，besidesthese，therearebivalvesandunivalves。Thosewithturbinateshellsmay，indeed，afteracertainfashionbesaidtoresemblebivalves。Fortheyallfromtheirverybirthhaveanoperculumtoprotectthatpartoftheirbodywhichisexposedtoview。ThisisthecasewiththePurpuras，withWhelks，withtheNerites，andthelike。Wereitnotforthis，thepartwhichisundefendedbytheshellwouldbeveryliabletoinjurybycollisionwithexternalobjects。Theunivalvesalsoarenotwithoutprotection。Forontheirdorsalsurfacetheyhaveashell，andbytheundersurfacetheyattachthemselvestotherocks，andsoafteramannerbecomebivalved，therockrepresentingthesecondvalve。OfthesetheanimalsknownasLimpetsareanexample。Thebivalves，scallopsandmussels，forinstance，areprotectedbythepowertheyhaveofclosingtheirvalves；andtheTurbinatabytheoperculumjustmentioned，whichtransformsthem，asitwere，cromunivalvesintobivalves。Butofallthereisnonesoperfectlyprotectedasthesea-urchin。Forherethereisaglobularshellwhichenclosesthebodycompletely，andwhichis，moreover，setwithsharpspines。Thispeculiaritydistinguishesthesea-urchinfromallotherTestacea，ashasalreadybeenmentioned。
ThestructureoftheTestaceaandoftheCrustaceaisexactlythereverseofthatoftheCephalopoda。Forinthelatterthefleshysubstanceisontheoutsideandtheearthysubstancewithin，whereasintheformerthesoftpartsareinsideandthehardpartwithout。
Inthesea-urchin，however，thereisnofleshypartwhatsoever。
AlltheTestaceathen，thosethathavenotbeenmentionedaswellasthosethathave，agreeasstatedinpossessingamouthwiththetongue-likebody，astomach，andaventforexcrement，buttheydifferfromeachotherinthepositionsandproportionsoftheseparts。Thedetails，however，ofthesedifferencesmustbelookedforintheResearchesconcerningAnimalsandthetreatisesonAnatomy。Forwhiletherearesomepointswhichcanbemadeclearbyverbaldescription，thereareotherswhicharemoresuitedforoculardemonstration。
PeculiaramongtheTestaceaarethesea-urchinsandtheanimalsknownasTethya（Ascidians）。Thesea-urchinshavefiveteeth，andinthecentreofthesethefleshybodywhichiscommontoalltheanimalswehavebeendiscussing。Immediatelyafterthiscomesagullet，andthenthestomach，dividedintoanumberofseparatecompartments，whichlooklikesomanydistinctstomachs；forthecavitiesareseparateandallcontainabundantresidualmatter。Theyareall，however，connectedwithoneandthesameoesophagus，andtheyallendinoneandthesameexcrementalvent。Thereisnothingbesidesthestomachofafleshycharacter，ashasalreadybeenstated。Allthatcanbeseenaretheso-calledova，ofwhichthereareseveral，containedeachinaseparatemembrane，andcertainblackbodieswhichhavenoname，andwhich，beginningattheanimal’smouth，arescatteredrounditsbodyhereandtherepromiscuously。Thesesea-urchinsarenotallofonespecies，butthereareseveraldifferentkinds，andinallofthemthepartsmentionedaretobefound。Itisnot，however，ineverykindthattheso-calledovaareedible。Neitherdotheseattaintoanysizeinanyotherspeciesthanthatwithwhichweareallfamiliar。AsimilardistinctionmaybemadegenerallyinthecaseofallTestacea。Forthereisagreatdifferenceintheediblequalitiesofthefleshofdifferentkinds；
andinsome，moreover，theresidualsubstanceknownasthemeconisgoodforfood，whileinothersitisuneatable。Thismeconintheturbinatedgeneraislodgedinthespiralpartoftheshell，whileinunivalves，suchaslimpets，itoccupiesthefundus，andinbivalvesisplacednearthehinge，theso-calledovumlyingontheright；whileontheoppositesideisthevent。Theformerisincorrectlytermedovum，foritmerelycorrespondstowhatinwell-fedsanguineousanimalsisfat；andthusitisthatitmakesitsappearanceinTestaceaatthoseseasonsoftheyearwhentheyareingoodcondition，namely，springandautumn。FornoTestaceacanabideextremesoftemperature，andtheyarethereforeinevilplightinseasonsofgreatcoldorheat。Thisisclearlyshownbywhatoccursinthecaseofthesea-urchins。Forthoughtheovaaretobefoundintheseanimalsevendirectlytheyareborn，yettheyacquireagreatersizethanusualatthetimeoffullmoon；not，assomethink，becausesea-urchinseatmoreatthatseason，butbecausethenightsarethenwarmer，owingtothemoonlight。Forthesecreaturesarebloodless，andsoareunabletostandcoldandrequirewarmth。Thereforeitisthattheyarefoundinbetterconditioninsummerthanatanyotherseason；
andthisallovertheworldexceptinginthePyrrheantidalstrait。
Therethesea-urchinsflourishaswellinwinterasinsummer。Butthereasonforthisisthattheyhaveagreaterabundanceoffoodinthewinter，becausethefishdesertthestraitatthatseason。
Thenumberoftheovaisthesameinallsea-urchins，andisanoddone。Fortherearefiveova，justastherearealsofiveteethandfivestomachs；andtheexplanationofthisistobefoundinthefactthattheso-calledovaarenotreallyova，butmerely，aswassaidbefore，theresultoftheanimal’swell-fedcondition。Oystersalsohaveaso-calledovum，correspondingincharactertothatofthesea-urchins，butexistingonlyononesideoftheirbody。Nowinasmuchasthesea-urchinisofasphericalform，andnotmerelyasinglediskliketheoyster，andinvirtueofitssphericalshapeisthesamefromwhateversideitbeexamined，itsovummustnecessarilybeofacorrespondingsymmetry。Forthesphericalshapehasnottheasymmetryofthedisk-shapedbodyoftheoysters。Forinalltheseanimalstheheadiscentral，butinthesea-urchintheso-calledovumisabove[andsymmetrical，whileintheoysteritisonlyoneside]。Nowthenecessarysymmetrywouldbeobservedweretheovumtoformacontinuousring。Butthismaynotbe。ForitwouldbeinoppositiontowhatprevailsinthewholetribeofTestacea；forinalltheovumisdiscontinuous，andinallexceptingthesea-urchinsasymmetrical，beingplacedonlyononesideofthebody。Owingthentothisnecessarydiscontinuityoftheovum，whichbelongstothesea-urchinasamemberoftheclass，andowingtothesphericalshapeofitsbody，whichisitsindividualpeculiarity，thisanimalcannotpossiblyhaveanevennumberofova。Forweretheyanevennumber，theywouldhavetobearrangedexactlyoppositetoeachother，inpairs，soastokeepthenecessarysymmetry；oneovumofeachpairbeingplacedatoneend，theotherovumattheotherendofatransversediameter。ThisagainwouldviolatetheuniversalprovisioninTestacea。Forbothintheoystersandinthescallopswefindtheovumonlyononesideofthecircumference。Thenumberthenoftheovamustbeuneven，threeforinstance，orfive。Butiftherewereonlythreetheywouldbemuchtoofarapart；while，ifthereweremorethanfive，theywouldcometoformacontinuousmass。Theformerarrangementwouldbedisadvantageoustotheanimal，thelatteranimpossibility。Therecanthereforebeneithermorenorlessthanfive。Forthesamereasonthestomachisdividedintofiveparts，andthereisacorrespondingnumberofteeth。Forseeingthattheovarepresenteachofthemakindofbodyfortheanimal，theirdispositionmustconformtothatofthestomach，seeingthatitisfromthisthattheyderivethematerialfortheirgrowth。Nowiftherewereonlyonestomach，eithertheovawouldbetoofarofffromit，oritwouldbesobigastofillupthewholecavity，andthesea-urchinwouldhavegreatdifficultyinmovingaboutandfindingduenourishmentforitsrepletion。Asthentherearefiveintervalsbetweenthefiveova，soarethereofnecessityfivedivisionsofthestomach，oneforeachinterval。Soalso，andonlikegrounds，therearefiveteeth。Fornatureisthusenabledtoallottoeachstomachalcompartmentandovumitsseparateandsimilartooth。
These，then，arethereasonswhythenumberofovainthesea-urchinisanoddone，andwhythatoddnumberisfive。Insomesea-urchinstheovaareexcessivelysmall，inothersofconsiderablesize，theexplanationbeingthatthelatterareofawarmerconstitution，andsoareabletoconcocttheirfoodmorethoroughly；whileintheformerconcoctionislessperfect，sothatthestomachisfoundfullofresidualmatter，whiletheovaaresmallanduneatable。Thoseofawarmerconstitutionare，moreover，invirtueoftheirwarmthmoregiventomotion，sothattheymakeexpeditionsinsearchoffood，insteadofremainingstationaryliketherest。Asevidenceofthis，itwillbefoundthattheyalwayshavesomethingorotherstickingtotheirspines，asthoughtheymovedmuchabout；fortheyusetheirspinesasfeet。
TheAscidiansdifferbutslightlyfromplants，andyethavemoreofananimalnaturethanthesponges，whicharevirtuallyplantsandnothingmore。Fornaturepassesfromlifelessobjectstoanimalsinsuchunbrokensequence，interposingbetweenthembeingswhichliveandyetarenotanimals，thatscarcelyanydifferenceseemstoexistbetweentwoneighbouringgroupsowingtotheircloseproximity。
Asponge，then，asalreadysaid，intheserespectscompletelyresemblesaplant，thatthroughoutitslifeitisattachedtoarock，andthatwhenseparatedfromthisitdies。Slightlydifferentfromthespongesaretheso-calledHolothuriasandthesea-lungs，asalsosundryothersea-animalsthatresemblethem。Forthesearefreeandunattached。Yettheyhavenofeeling，andtheirlifeissimplythatofaplantseparatedfromtheground。Forevenamongland-plantstherearesomethatareindependentofthesoil，andthatspringupandgrow，eitheruponotherplants，orevenentirelyfree。Such，forexample，istheplantwhichisfoundonParnassus，andwhichsomecalltheEpipetrum。Thisyoumayhanguponapeganditwillyetliveforaconsiderabletime。Sometimesitisamatterofdoubtwhetheragivenorganismshouldbeclassedwithplantsorwithanimals。TheAscidians，forinstance，andthelikesofarresembleplantsasthattheyneverlivefreeandunattached，but，ontheotherhand，inasmuchastheyhaveacertainflesh-likesubstance，theymustbesupposedtopossesssomedegreeofsensibility。
AnAscidianhasabodydividedbyasingleseptumandwithtwoorifices，onewhereittakesinthefluidmatterthatministerstoitsnutrition，theotherwhereitdischargesthesurplusofunusedjuice，forithasnovisibleresidualsubstance，suchashavetheotherTestacea。ThisisitselfaverystrongjustificationforconsideringanAscidian，andanythingelsetheremaybeamonganimalsthatresemblesit，tobeofavegetablecharacter；forplantsalsoneverhaveanyresiduum。AcrossthemiddleofthebodyoftheseAscidiansthererunsathintransversepartition，andhereitisthatwemayreasonablysupposethepartonwhichlifedependstobesituated。
TheAcalephae，orSea-nettles，astheyarevariouslycalled，arenotTestaceaatall，butlieoutsidetherecognizedgroups。Theirconstitution，likethatoftheAscidians，approximatesthemononesidetoplants，ontheothertoanimals。Forseeingthatsomeofthemcandetachthemselvesandcanfastenupontheirfood，andthattheyaresensibleofobjectswhichcomeincontactwiththem，theymustbeconsideredtohaveananimalnature。Thelikeconclusionfollowsfromtheirusingtheasperityoftheirbodiesasaprotectionagainsttheirenemies。But，ontheotherhand，theyarecloselyalliedtoplants，firstlybytheimperfectionoftheirstructure，secondlybytheirbeingabletoattachthemselvestotherocks，whichtheydowithgreatrapidity，andlastlybytheirhavingnovisibleresiduumnotwithstandingthattheypossessamouth。
VerysimilaragaintotheAcalephaearetheStarfishes。Forthesealsofastenontheirprey，andsuckoutitsjuices，andthusdestroyavastnumberofoysters。AtthesametimetheypresentacertainresemblancetosuchoftheanimalswehavedescribedastheCephalopodaandCrustacea，inasmuchastheyarefreeandunattached。
ThesamemayalsobesaidoftheTestacea。
Such，then，isthestructureofthepartsthatministertonutritionandwhicheveryanimalmustpossess。Butbesidestheseorgansitisquiteplainthatineveryanimaltheremustbesomepartorotherwhichshallbeanalogoustowhatinsanguineousanimalsisthepresidingseatofsensation。Whetherananimalhasorhasnotblood，itcannotpossiblybewithoutthis。IntheCephalopodathispartconsistsofafluidsubstancecontainedinamembrane，throughwhichrunsthegulletonitswaytothestomach。Itisattachedtothebodyrathertowardsitsdorsalsurface，andbysomeiscalledthemytis。JustsuchanotherorganisfoundalsointheCrustaceaandtheretooisknownbythesamename。Thispartisatoncefluidandcorporealand，asbeforesaid，istraversedbythegullet。Forhadthegulletbeenplacedbetweenthemytisandthedorsalsurfaceoftheanimal，thehardnessofthebackwouldhaveinterferedwithitsduedilatationintheactofdeglutition。Ontheoutersurfaceofthemytisrunstheintestine；andincontactwiththislatterisplacedtheink-bag，sothatitmayberemovedasfaraspossiblefromthemouthanditsobnoxiousfluidbekeptatadistancefromthenoblerandsovereignpart。Thepositionofthemytisshowsthatitcorrespondstotheheartofsanguineousanimals；foritoccupiestheself-sameplace。Thesameisshownbythesweetnessofitsfluid，whichhasthecharacterofconcoctedmatterandresemblesblood。
IntheTestaceathepresidingseatofsensationisinacorrespondingposition，butislesseasilymadeout。Itshould，however，alwaysbelookedforinsomemidwayposition；namely，insuchTestaceaasarestationary，midwaybetweenthepartbywhichfoodistakeninandthechannelthroughwhicheithertheexcrementorthespermaticfluidisvoided，and，inthosespecieswhicharecapableoflocomotion，invariablymidwaybetweentherightandleftsides。
InInsectsthisorgan，whichistheseatofsensation，lies，aswasstatedinthefirsttreatise，betweentheheadandthecavitywhichcontainsthestomach。Inmostofthemitconsistsofasinglepart；butinothers，forinstanceinsuchashavelongbodiesandresembletheJuli（Millipedes），itismadeupofseveralparts，sothatsuchinsectscontinuetoliveaftertheyhavebeencutinpieces。
Fortheaimofnatureistogivetoeachanimalonlyonesuchdominantpart；andwhensheisunabletocarryoutthisintentionshecausestheparts，thoughpotentiallymany，toworktogetheractuallyasone。Thisismuchmoreclearlymarkedinsomeinsectsthaninothers。
Thepartsconcernedinnutritionarenotalikeinallinsects，butshowconsiderablediversity。Thussomehavewhatiscalledastinginthemouth，whichisakindofcompoundinstrumentthatcombinesinitselfthecharacterofatongueandoflips。Inothersthathavenosuchinstrumentinfrontthereisapartinsidethemouththatanswersthesamesensorypurposes。Immediatelyafterthemouthcomestheintestine，whichisneverwantinginanyinsect。Thisrunsinastraightlineandwithoutfurthercomplicationtothevent；
occasionally，however，ithasaspiralcoil。Thereare，moreover，someinsectsinwhichastomachsucceedstothemouth，andisitselfsucceededbyaconvolutedintestine，sothatthelargerandmorevoraciousinsectsmaybeenabledtotakeinamoreabundantsupplyoffood。MorecuriousthananyaretheCicadae。Forherethemouthandthetongueareunitedsoastoformasinglepart，throughwhich，asthrougharoot，theinsectsucksupthefluidsonwhichitlives。
Insectsarealwayssmalleaters，notsomuchbecauseoftheirdiminutivesizeasbecauseoftheircoldtemperament。Foritisheatwhichrequiressustenance；justasitisheatwhichspeedilyconcoctsit。Butcoldrequiresnosustenance。InnoinsectsisthissoconspicuousasintheseCicadae。Fortheyfindenoughtoliveoninthemoisturewhichisdepositedfromtheair。SoalsodotheEphemerathatarefoundabouttheBlacksea。Butwhiletheselatteronlyliveforasingleday，theCicadaesubsistonsuchfoodforseveraldays，thoughstillnotmany。
Wehavenowdonewiththeinternalpartsofanimals，andmustthereforereturntotheconsiderationoftheexternalpartswhichhavenotyetbeendescribed。Itwillbebettertochangeourorderofexpositionandbeginwiththeanimalswehavejustbeendescribing，sothatproceedingfromthese，whichrequirelessdiscussion，ouraccountmayhavemoretimetospendontheperfectkindsofanimals，thosenamelythathaveblood。
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WewillbeginwithInsects。Theseanimals，thoughtheypresentnogreatmultiplicityofparts，arenotwithoutdiversitieswhencomparedwitheachother。Theyareallmanyfooted；theobjectofthisbeingtocompensatetheirnaturalslownessandfrigidity，andgivegreateractivitytotheirmotions。Accordinglywefindthatthosewhich，asthe（Millipedes），havelongbodies，andarethereforethemostliabletorefrigeration，havealsothegreatestnumberoffeet。Again，thebodyintheseanimalsisinsected-thereasonforthisbeingthattheyhavenotgotonevitalcentrebutmany-andthenumberoftheirfeetcorrespondstothatoftheinsections。
Shouldthefeetfallshortofthis，theirdeficiencyiscompensatedbythepowerofflight。Ofsuchflyinginsectssomeliveawanderinglife，andareforcedtomakelongexpeditionsinsearchoffood。Thesehaveabodyoflightweight，andfourfeathers，twooneitherside，tosupportit。Sucharebeesandtheinsectsakintothem。When，however，suchinsectsareofverysmallbulk，theirfeathersarereducedtotwo，asisthecasewithflies。Insectswithheavybodiesandofstationaryhabits，thoughnotpolypterousinthesamewayasbees，yethavesheathstotheirfeatherstomaintaintheirefficiency。SucharetheMelolonthaeandthelike。Fortheirstationaryhabitsexposetheirfeatherstomuchgreaterrisksthanarerunbythoseofinsectsthataremoreconstantlyinflight，andonthisaccounttheyareprovidedwiththisprotectingshield。Thefeatherofaninsecthasneitherbarbsnorshaft。For，thoughitiscalledafeather，itisnofeatheratall，butmerelyaskin-likemembranethat，owingtoitsdryness，necessarilybecomesdetachedfromthesurfaceofthebody，asthefleshysubstancegrowscold。
Theseanimalsthenhavetheirbodiesinsected，notonlyforthereasonsalreadyassigned，butalsotoenablethemtocurlroundinsuchamannerasmayprotectthemfrominjury；forsuchinsectsashavelongbodiescanrollthemselvesup，whichwouldbeimpossiblewereitnotfortheinsections；andthosethatcannotdothiscanyetdrawtheirsegmentsupintotheinsectedspaces，andsoincreasethehardnessoftheirbodies。Thiscanbefeltquiteplainlybyputtingthefingerononeoftheinsects，forinstance，knownasCanthari。Thetouchfrightenstheinsect，anditremainsmotionless，whileitsbodybecomeshard。Thedivisionofthebodyintosegmentsisalsoanecessaryresultoftherebeingseveralsupremeorgansinplaceofone；andthisagainisapartoftheessentialconstitutionofinsects，andisacharacterwhichapproximatesthemtoplants。Forasplants，thoughcutintopieces，canstilllive，soalsocaninsects。Thereis，however，thisdifferencebetweenthetwocases，thattheportionsofthedividedinsectliveonlyforalimitedtime，whereastheportionsoftheplantliveonandattaintheperfectformofthewhole，sothatfromonesingleplantyoumayobtaintwoormore。
Someinsectsarealsoprovidedwithanothermeansofprotectionagainsttheirenemies，namelyasting。Insomethisisinfront，connectedwiththetongue，inothersbehindattheposteriorend。
Forjustastheorganofsmellinelephantsanswersseveraluses，servingalikeasaweaponandforpurposesofnutrition，sodoesalsothesting，whenplacedinconnexionwiththetongue，asinsomeinsects，answermorethanoneend。Foritistheinstrumentthroughwhichtheyderivetheirsensationsoffood，aswellasthatwithwhichtheysuckitupandbringittothemouth。Suchoftheseinsectsashavenoanteriorstingareprovidedwithteeth，whichserveinsomeofthemforbitingthefood，andinothersforitsprehensionandconveyancetothemouth。Sucharetheiruses，forinstance，inantsandallthevariouskindsofbees。Asfortheinsectsthathaveastingbehind，thisweaponisgiventhembecausetheyareofafiercedisposition。Insomeofthemthestingislodgedinsidethebody，inbees，forexample，andwasps。Fortheseinsectsaremadeforflight，andweretheirstingexternalandofdelicatemakeitwouldsoongetspoiled；andif，ontheotherhand，itwereofthickerbuild，asinscorpions，itsweightwouldbeanincumbrance。Asforscorpionsthatliveonthegroundandhaveatail，theirstingmustbesetuponthis，asotherwiseitwouldbeofnouseasaweapon。Dipterousinsectsneverhaveaposteriorsting。Fortheveryreasonoftheirbeingdipterousisthattheyaresmallandweak，andthereforerequirenomorethantwofeatherstosupporttheirlightweight；andthesamereasonwhichreducestheirfeatherstotwocausestheirstingtobeinfront；fortheirstrengthisnotsufficienttoallowthemtostrikeefficientlywiththehinderpartofthebody。Polypterousinsects，ontheotherhand，areofgreaterbulk-indeeditisthiswhichcausesthemtohavesomanyfeathers；andtheirgreatersizemakesthemstrongerintheirhinderparts。Thestingofsuchinsectsisthereforeplacedbehind。Nowitisbetter，whenpossible，thatoneandthesameinstrumentshallnotbemadetoserveseveraldissimilaruses；butthatthereshallbeoneorgantoserveasaweapon，whichcanthenbeverysharp，andadistinctonetoserveasatongue，whichcanthenbeofspongytextureandfittoabsorbnutriment。Whenever，therefore，natureisabletoprovidetwoseparateinstrumentsfortwoseparateuses，withouttheonehamperingtheother，shedoesso，insteadofactinglikeacoppersmithwhoforcheapnessmakesaspitandlampholderinone。Itisonlywhenthisisimpossiblethatsheusesoneorganforseveralfunctions。
Theanteriorlegsareinsomecaseslongerthantheothers，thattheymayservetowipeawayanyforeignmatterthatmaylodgeontheinsect’seyesandobstructitssight，whichalreadyisnotverydistinctowingtotheeyesbeingmadeofahardsubstance。Fliesandbeesandthelikemaybeconstantlyseenthusdressingthemselveswithcrossedforelegs。Oftheotherlegs，thehinderarebiggerthanthemiddlepair，bothtoaidinrunningandalsothattheinsect，whenittakesflight，mayspringmoreeasilyfromtheground。Thisdifferenceisstillmoremarkedinsuchinsectsasleap，inlocustsforinstance，andinthevariouskindsoffleas。Forthesefirstbendandthenextendthelegs，and，bydoingso，arenecessarilyshotupfromtheground。Itisonlythe。hindlegsoflocusts，andnotthefrontones，thatresemblethesteeringoarsofaship。Forthisrequiresthatthejointshallbedeflectedinwards，andsuchisneverthecasewiththeanteriorlimbs。Thewholenumberoflegs，includingthoseusedinleaping，issixinalltheseinsects。
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IntheTestaceathebodyconsistsofbutfewparts，thereasonbeingthattheseanimalsliveastationarylife。Forsuchanimalsasmovemuchaboutmustofnecessityhavemorenumerouspartsthansuchasremainquiet；fortheiractivitiesaremany，andthemorediversifiedthemovementsthegreaterthenumberoforgansrequiredtoeffectthem。SomespeciesofTestaceaareabsolutelymotionless，andothersnotquitebutnearlyso。Nature，however，hasprovidedthemwithaprotectioninthehardnessoftheshellwithwhichshehasinvestedtheirbody。Thisshell，asalreadyhasbeensaid，mayhaveonevalve，ortwovalves，orbeturbinate。Inthelattercaseitmaybeeitherspiral，asinwhelks，ormerelyglobular，asinsea-urchins。
Whenithastwovalves，thesemaybegaping，asinscallopsandmussels，wherethevalvesareunitedtogetherononesideonly，soastoopenandshutontheother；ortheymaybeunitedtogetheronbothsides，asintheSolens（razor-fishes）。InallcasesaliketheTestaceahave，likeplants，theheaddownwards。Thereasonforthisis，thattheytakeintheirnourishmentfrombelow，justasdoplantswiththeirroots。Thustheunderpartscomeinthemtobeabove，andtheupperpartstobebelow。Thebodyisenclosedinamembrane，andthroughthistheanimalfiltersfluidfreefromsaltandabsorbsitsnutriment。Inallthereisahead；butnoneoftheparts，exceptingthisrecipientoffood，hasanydistinctivename。
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AlltheCrustaceacancrawlaswellasswim，andaccordinglytheyareprovidedwithnumerousfeet。Therearefourmaingenera，viz。
theCarabi，astheyarecalled，theAstaci，theCarides，andtheCarcini。Ineachofthesegenera，again，therearenumerousspecies，whichdifferfromeachothernotonlyasregardsshape，butalsoveryconsiderablyasregardssize。For，whileinsomespeciestheindividualsarelarge，inotherstheyareexcessivelyminute。TheCarcinoidandCaraboidCrustacearesembleeachotherinpossessingclaws。Theseclawsarenotforlocomotion，buttoserveinplaceofhandsforseizingandholdingobjects；andtheyarethereforebentintheoppositedirectiontothefeet，beingsotwistedastoturntheirconvexitytowardsthebody，whiletheirfeetturntowardsittheirconcavity。Forinthispositiontheclawsarebestsuitedforlayingholdofthefoodandcarryingittothemouth。ThedistinctionbetweentheCarabiandtheCarcini（Crabs）consistsintheformerhavingatailwhilethelatterhavenone。FortheCarabiswimaboutandatailisthereforeofusetothem，servingfortheirpropulsionlikethebladeofanoar。ButitwouldbeofnousetotheCrabs；fortheseanimalslivehabituallyclosetotheshore，andcreepintoholesandcorners。Insuchofthemasliveoutatsea，thefeetaremuchlessadaptedforlocomotionthanintherest，becausetheyarelittlegiventomovingaboutbutdependforprotectionontheirshell-likecovering。TheMaiaeandthecrabsknownasHeracleoticareexamplesofthis；thelegsintheformerbeingverythin，inthelatterveryshort。
Theveryminutecrabsthatarefoundamongthesmallfryatthebottomofthenethavetheirhindermostfeetflattenedoutintothesemblanceoffinsoroar-blades，soastohelptheanimalinswimming。
TheCaridesaredistinguishedfromtheCarcinoidspeciesbythepresenceofatail；andfromtheCaraboidsbytheabsenceofclaws。
Thisisexplainedbytheirlargenumberoffeet，onwhichhasbeenexpendedthematerialforthegrowthofclaws。Theirfeetagainarenumeroustosuittheirmodeofprogression，whichismainlybyswimming。
Ofthepartsontheventralsurface，thoseneartheheadareinsomeoftheseanimalsformedlikegills，fortheadmissionanddischargeofwater；whilethepartslowerdowndifferinthetwosexes。ForinthefemaleCarabithesearemorelaminarthaninthemales，andinthefemalecrabstheflapisfurnishedwithhairierappendages。Thisgivesamplerspaceforthedisposaloftheova，whichthefemalesretaininthesepartsinsteadoflettingthemgofree，asdofishesandallotheroviparousanimals。IntheCarabiandintheCrabstherightclawisinvariablythelargerandthestronger。Foritisnaturaltoeveryanimalinactiveoperationstousethepartsonitsrightsideinpreferencetothoseonitsleft；andnature，indistributingtheorgans，invariablyassignseach，eitherexclusivelyorinamoreperfectcondition，tosuchanimalsascanuseit。Soitiswithtusks，andteeth，andhorns，andspurs，andallsuchdefensiveandoffensiveweapons。
IntheLobstersaloneitisamatterofchancewhichclawisthelarger，andthisineithersex。Clawstheymusthave，becausetheybelongtoagenusinwhichthisisaconstantcharacter；buttheyhavetheminthisindeterminateway，owingtoimperfectformationandtotheirnotusingthemfortheirnaturalpurpose，butforlocomotion。
Foradetailedaccountoftheseveralpartsoftheseanimals，oftheirpositionandtheirdifferences，thosepartsbeingalsoincludedwhichdistinguishthesexes，referencemustbemadetothetreatisesonAnatomyandtotheResearchesconcerningAnimals。
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WecomenowtotheCephalopoda。Theirinternalorganshavealreadybeendescribedwiththoseofotheranimals。Externallythereisthetrunkofthebody，notdistinctlydefined，andinfrontofthistheheadsurroundedbyfeet，whichformacircleaboutthemouthandteeth，andaresetbetweentheseandtheeyes。Nowinallotheranimalsthefeet，ifthereareany，aredisposedinoneoftwoways；
eitherbeforeandbehindoralongthesides，thelatterbeingtheplaninsuchofthem，forinstance，asarebloodlessandhavenumerousfeet。ButintheCephalopodathereisapeculiararrangement，differentfromeitherofthese。Fortheirfeetareallplacedatwhatmaybecalledtheforeend。Thereasonforthisisthatthehindpartoftheirbodyhasbeendrawnupclosetotheforepart，asisalsothecaseintheturbinatedTestacea。FortheTestacea，whileinsomepointstheyresembletheCrustacea，inothersresembletheCephalopoda。Theirearthymatterisontheoutside，andtheirfleshysubstancewithin。SofartheyareliketheCrustacea。ButthegeneralplanoftheirbodyisthatoftheCephalopoda；and，thoughthisistrueinacertaindegreeofalltheTestacea，itismoreespeciallytrueofthoseturbinatedspeciesthathaveaspiralshell。Ofthisgeneralplan，commontothetwo，wewillspeakpresently。Butletusfirstconsiderthecaseofquadrupedsandofman，wherethearrangementisthatofastraightline。LetAattheupperendofsuchalinebesupposedtorepresentthemouth，thenB
thegullet，andCthestomach，andtheintestinetorunfromthisCtotheexcrementalventwhereDisinscribed。Suchistheplaninsanguineousanimals；androundthisstraightlineasanaxisaredisposedtheheadandso-calledtrunk；theremainingparts，suchastheanteriorandposteriorlimbs，havingbeensuperaddedbynature，merelytoministertotheseandforlocomotion。
IntheCrustaceaalsoandinInsectsthereisatendencytoasimilararrangementoftheinternalpartsinastraightline；thedistinctionbetweenthesegroupsandthesanguineousanimalsdependingondifferencesoftheexternalorganswhichministertolocomotion。
ButtheCephalopodaandtheturbinatedTestaceahaveincommonanarrangementwhichstandsincontrastwiththis。Forherethetwoextremitiesarebroughttogetherbyacurve，asifoneweretobendthestraightlinemarkedEuntilDcameclosetoSuch，then，isthedispositionoftheinternalparts；androundthese，intheCephalopoda，isplacedthesac（inthePoulpsalonecalledahead），and，intheTestacea，theturbinateshellwhichcorrespondstothesac。Thereis，infact，onlythisdifferencebetweenthem，thattheinvestingsubstanceoftheCephalopodaissoftwhiletheshelloftheTestaceaishard，naturehavingsurroundedtheirfleshypartwiththishardcoatingasaprotectionbecauseoftheirlimitedpoweroflocomotion。Inbothclasses，owingtothisarrangementoftheinternalorgans，theexcrementisvoidednearthemouth；atapointbelowthisorificeintheCephalopoda，andintheTurbinataononesideofit。
Such，then，istheexplanationofthepositionofthefeetintheCephalopoda，andofthecontrasttheypresenttootheranimalsinthismatter。Thearrangement，however，intheSepiasandtheCalamariesisnotpreciselythesameasinthePoulps，owingtotheformerhavingnoothermodeofprogressionthanbyswimming，whilethelatternotonlyswimbutcrawl。Forintheformersixofthefeetareabovetheteethandsmall，theouteroneoneithersidebeingthebiggest；
whiletheremainingtwo，whichmakeupthetotalweight，arebelowthemouthandarethebiggestofall，justasthehindlimbsinquadrupedsarestrongerthantheforelimbs。Foritisthesethathavetosupporttheweight，andtotakethemainpartinlocomotion。Andtheoutertwooftheuppersixarebiggerthanthepairwhichintervenebetweenthemandtheuppermostofall，becausetheyhavetoassistthelowermostpairintheiroffice。InthePoulps，ontheotherhand，thefourcentralfeetarethebiggest。Again，thoughthenumberoffeetisthesameinalltheCephalopoda，namelyeight，theirlengthvariesindifferentkinds，beingshortintheSepiasandtheCalamaries，butgreaterinthePoulps。Forintheselatterthetrunkofthebodyisofsmallbulk，whileintheformeritisofconsiderablesize；
andsointheonecasenaturehasusedthematerialssubtractedfromthebodytogivelengthtothefeet，whileintheothershehasactedinpreciselytheoppositeway，andhasgiventothegrowthofthebodywhatshehasfirsttakenfromthefeet。ThePoulps，then，owingtothelengthoftheirfeet，cannotonlyswimbutcrawl，whereasintheothergenerathefeetareuselessforthelattermodeofprogression，beingsmallwhilethebulkofthebodyisconsiderable。Theseshortfeetwouldnotenabletheirpossessorstoclingtotherocksandkeepthemselvesfrombeingtornoffbythewaveswhentheserunhighintimesofstorm；neitherwouldtheyservetolayholdofobjectsatallremoteandbringthemin；but，tosupplythesedefects，theanimalisfurnishedwithtwolongproboscises，bywhichitcanmooritselfandrideatanchorlikeashipinroughweather。Thesesameprocessesservealsotocatchpreyatadistanceandtobringittothemouth。TheyaresousedbyboththeSepiasandtheCalamaries。InthePoulpsthefeetarethemselvesabletoperformtheseoffices，andthereareconsequentlynoproboscises。Proboscisesandtwiningtentacles，withacetabulasetuponthem，actinthesamewayandhavethesamestructureasthoseplaitedinstrumentswhichwereusedbyphysiciansofoldtoreducedislocationsofthefingers。Likethesetheyaremadebytheinterlacingoftheirfibres，andtheyactbypullinguponpiecesoffleshandyieldingsubstances。Fortheplaitedfibresencircleanobjectinaslackenedcondition，andwhentheyareputonthestretchtheygraspandclingtightlytowhateveritmaybethatisincontactwiththeirinnersurface。Since，then，theCephalopodahavenootherinstrumentswithwhichtoconveyanythingtothemselvesfromwithout，thaneithertwiningtentacles，asinsomespecies，orproboscisesasinothers，theyareprovidedwiththesetoserveashandsforoffenceanddefenceandothernecessaryuses。
TheacetabulaaresetindoublelineinalltheCephalopodaexceptinginonekindofpoulp，wherethereisbutasinglerow。Thelengthandtheslimnesswhichispartofthenatureofthiskindofpoulpexplaintheexception。Foranarrowspacecannotpossiblyadmitofmorethanasinglerow。Thisexceptionalcharacter，then，belongstothem，notbecauseitisthemostadvantageousarrangement，butbecauseitisthenecessaryconsequenceoftheiressentialspecificconstitution。
Inalltheseanimalsthereisafin，encirclingthesac。InthePoulpsandtheSepiasthisfinisunbrokenandcontinuous，asisalsothecaseinthelargercalamariesknownasTeuthi。Butinthesmallerkind，calledTeuthides，thefinisnotonlybroaderthanintheSepiasandthePoulps，whereitisverynarrow，but，moreover，doesnotencircletheentiresac，butonlybeginsinthemiddleoftheside。Theuseofthisfinistoenabletheanimaltoswim，andalsotodirectitscourse。Itacts，thatis，liketherump-feathersinbirds，orthetail-fininfishes。InnoneisitsosmallorsoindistinctasinthePoulps。Forinthesethebodyisofsmallbulkandcanbesteeredbythefeetsufficientlywellwithoutotherassistance。
TheInsects，theCrustacea，theTestacea，andtheCephalopoda，havenowbeendealtwithinturn；andtheirpartshavebeendescribed，whetherinternalorexternal。
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Wemustnowgobacktotheanimalsthathaveblood，andconsidersuchoftheirparts，alreadyenumerated，aswerebeforepassedover。
Wewilltaketheviviparousanimalsfirst，and，wehavedonewiththese，willpassontotheoviparous，andtreatoftheminlikemanner。
Thepartsthatborderonthehead，andonwhatisknownastheneckandthroat，havealreadybeentakenintoconsideration。Allanimalsthathavebloodhaveahead；whereasinsomebloodlessanimals，suchascrabs，thepartwhichrepresentsaheadisnotclearlydefined。Astotheneck，itispresentinalltheVivipara，butonlyinsomeoftheOvipara；forwhilethosethathavealungalsohaveaneck，thosethatdonotinhaletheouterairhavenone。Theheadexistsmainlyforthesakeofthebrain。Foreveryanimalthathasbloodmustofnecessityhaveabrain；andmust，moreover，forreasonsalreadygiven，haveitplacedinanoppositeregiontotheheart。Buttheheadhasalsobeenchosenbynatureasthepartinwhichtosetsomeofthesenses；becauseitsbloodismixedinsuchsuitableproportionsastoensuretheirtranquillityandprecision，whileatthesametimeitcansupplythebrainwithsuchwarmthasitrequires。Thereisyetathirdconstituentsuperaddedtothehead，namelythepartwhichministerstotheingestionoffood。Thishasbeenplacedherebynature，becausesuchasituationaccordsbestwiththegeneralconfigurationofthebody。Forthestomachcouldnotpossiblybeplacedabovetheheart，seeingthatthisisthesovereignorgan；andifplacedbelow，asinfactitis，thenthemouthcouldnotpossiblybeplacedtherealso。Forthiswouldhavenecessitatedagreatincreaseinthelengthofthebody；andthestomach，moreover，wouldhavebeenremovedtoofarfromthesourceofmotionandofconcoction。
Thehead，then，existsforthesakeofthesethreeparts；whiletheneck，again，existsforthesakeofthewindpipe。Foritactsasadefencetothisandtotheoesophagus，encirclingthemandkeepingthemfrominjury。Inallotheranimalsthisneckisflexibleandcontainsseveralvertebrae；butinwolvesandlionsitcontainsonlyasinglebone。Fortheobjectofnaturewastogivetheseanimalsanorganwhichshouldbeserviceableinthewayofstrength，ratherthanonethatshouldbeusefulforanyoftheotherpurposestowhichnecksaresubservient。
Continuouswiththeheadandneckisthetrunkwiththeanteriorlimbs。Inmantheforelegsandforefeetarereplacedbyarmsandbywhatwecallhands。Forofallanimalsmanalonestandserect，inaccordancewithhisgodlikenatureandessence。Foritisthefunctionofthegod-liketothinkandtobewise；andnoeasytaskwerethisundertheburdenofaheavybody，pressingdownfromaboveandobstructingbyitsweightthemotionsoftheintellectandofthegeneralsense。When，moreover，theweightandcorporealsubstancebecomeexcessive，thebodymustofnecessityinclinetowardstheground。Insuchcasesthereforenature，inordertogivesupporttothebody，hasreplacedthearmsandhandsbyforefeet，andhasthusconvertedtheanimalintoaquadruped。For，aseveryanimalthatwalksmustofnecessityhavethetwohinderfeet，suchananimalbecomesaquadruped，itsbodyincliningdownwardsinfrontfromtheweightwhichitssoulcannotsustain。Forallanimals，manaloneexcepted，aredwarf-likeinform。Forthedwarf-likeisthatinwhichtheupperpartislarge，whilethatwhichbearstheweightandisusedinprogressionissmall。Thisupperpartiswhatwecallthetrunk，whichreachesfromthemouthtothevent。Inmanitisdulyproportionatetothepartbelow，anddiminishesmuchinitscomparativesizeasthemanattainstofullgrowth。Butinhisinfancythecontraryobtains，andtheupperpartsarelarge，whilethelowerpartissmall；sothattheinfantcanonlycrawl，andisunabletowalk；nay，atfirstcannotevencrawl，butremainswithoutmotion。Forallchildrenaredwarfsinshape，butceasetobesoastheybecomemen，fromthegrowthoftheirlowerpart；whereasinquadrupedsthereverseoccurs，theirlowerpartsbeinglargestinyouth，andadvanceofyearsbringingincreasedgrowthabove，thatisinthetrunk，whichextendsfromtherumptothehead。Thusitisthatcoltsarescarcely，ifatall，belowfull-grownhorsesinheight；andthatwhilestillyoungtheycantouchtheirheadswiththehindlegs，thoughthisisnolongerpossiblewhentheyareolder。Such，then，istheformofanimalsthathaveeitherasolidoraclovenhoof。Butsuchasarepolydactylousandwithouthorns，thoughtheytooareofdwarf-likeshape，aresoinalessdegree；andthereforethegreatergrowthofthelowerpartsascomparedwiththeupperisalsosmall，beingproportionatetothissmallerdeficiency。
Dwarf-likeagainistheraceofbirdsandfishes；andsoinfact，asalreadyhasbeensaid，iseveryanimalthathasblood。Thisisthereasonwhynootheranimalissointelligentasman。Forevenamongmenthemselvesifwecomparechildrenwithadults，orsuchadultsasareofdwarf-likeshapewithsuchasarenot，wefindthat，whateverothersuperioritytheformermaypossess，theyareatanyratedeficientascomparedwiththelatterinintelligence。Theexplanation，asalreadystated，isthattheirpsychicalprincipleiscorporeal，andmuchimpededinitsmotions。Letnowafurtherdecreaseoccurintheelevatingheat，andafurtherincreaseintheearthymatter，andtheanimalsbecomesmallerinbulk，andtheirfeetmorenumerous，untilatalaterstagetheybecomeapodous，andextendedfulllengthontheground。Then，byfurthersmallsuccessionsofchange，theycometohavetheirprincipalorganbelow；andatlasttheircephalicpartbecomesmotionlessanddestituteofsensation。
Thustheanimalbecomesaplant，thathasitsupperpartsdownwardsanditslowerpartsabove。Forinplantstherootsaretheequivalentsofmouthandhead，whiletheseedhasanoppositesignificance，foritisproducedaboveittheextremitiesofthetwigs。
Thereasonshavenowbeenstatedwhysomeanimalshavemanyfeet，someonlytwo，andothersnone；why，also，somelivingthingsareplantsandothersanimals；and，lastly，whymanaloneofallanimalsstandserect。Standingthuserect，manhasnoneedoflegsinfront，andintheirsteadhasbeenendowedbynaturewitharmsandhands。NowitistheopinionofAnaxagorasthatthepossessionofthesehandsisthecauseofmanbeingofallanimalsthemostintelligent。Butitismorerationaltosupposethathisendowmentwithhandsistheconsequenceratherthanthecauseofhissuperiorintelligence。Forthehandsareinstrumentsororgans，andtheinvariableplanofnatureindistributingtheorgansistogiveeachtosuchanimalascanmakeuseofit；natureactinginthismatterasanyprudentmanwoulddo。Foritisabetterplantotakeapersonwhoisalreadyaflute-playerandgivehimaflute，thantotakeonewhopossessesafluteandteachhimtheartofflute-playing。Fornatureaddsthatwhichislesstothatwhichisgreaterandmoreimportant，andnotthatwhichismorevaluableandgreatertothatwhichisless。
Seeingthenthatsuchisthebettercourse，andseeingalsothatofwhatispossiblenatureinvariablybringsaboutthebest，wemustconcludethatmandoesnotowehissuperiorintelligencetohishands，buthishandstohissuperiorintelligence。Forthemostintelligentofanimalsistheonewhowouldputthemostorganstouse；andthehandisnottobelookedonasoneorganbutasmany；foritis，asitwere，aninstrumentforfurtherinstruments。Thisinstrument，therefore，-thehand-ofallinstrumentsthemostvariouslyserviceable，hasbeengivenbynaturetoman，theanimalofallanimalsthemostcapableofacquiringthemostvariedhandicrafts。