Thekidneyswhentheyarepresentexistnotofactualnecessity，butasmattersofgreaterfinishandperfection。Forbytheirspecialcharactertheyaresuitedtoserveintheexcretionofthefluidwhichcollectsinthebladder。Inanimalsthereforewherethisfluidisveryabundantlyformed，theirpresenceenablesthebladdertoperformitsproperofficewithgreaterperfection。
Sincethenbothkidneysandbladderexistinanimalsforoneandthesamefunction，wemustnexttreatofthebladder，thoughinsodoingwedisregardthedueorderofsuccessioninwhichthepartsshouldbeenumerated。Fornotawordhasyetbeensaidofthemidriff，whichisoneofthepartsthatenvironthevisceraandthereforehastobeconsideredwiththem。
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Itisnoteveryanimalthathasabladder；thoseonlybeingapparentlyintendedbynaturetohaveone，whoselungcontainsblood。Tosuchitwasbutreasonablethatsheshouldgivethispart。
Forthesuperabundanceintheirlungofitsnaturalconstituentscausesthemtobethethirstiestofanimals，andmakesthemrequireamorethanordinaryquantitynotmerelyofsolidbutalsoofliquidnutriment。Thisincreasedconsumptionnecessarilyentailstheproductionofanincreasedamountofresidue；whichthusbecomestooabundanttobeconcoctedbythestomachandexcretedwithitsownresidualmatter。Theresidualfluidmustthereforeofnecessityhaveareceptacleofitsown；andthusitcomestopassthatallanimalswhoselungcontainsbloodareprovidedwithabladder。Thoseanimals，ontheotherhand，thatarewithoutalungofthischaracter，andthateitherdrinkbutsparinglyowingtotheirlungbeingofaspongytexture，orneverimbibefluidatallfordrinking’ssakebutonlyasnutriment，insectsforinstanceandfishes，andthataremoreovercladwithfeathersorscalesorscalyplates-alltheseanimals，owingtothesmallamountoffluidwhichtheyimbibe，andowingalsotosuchresidueastheremaybebeingconvertedintofeathersandthelike，areinvariablywithoutabladder。TheTortoises，whicharecomprisedamonganimalswithscalyplates，formtheonlyexception；andthisismerelyduetotheimperfectdevelopmentoftheirnaturalconformation；theexplanationofthematterbeingthatinthesea-tortoisesthelungisflesh-likeandcontainsblood，resemblingthelungoftheox，andthatintheland-tortoisesitisofdisproportionatelylargesize。Moreover，inasmuchasthecoveringwhichinveststhemisdenseandshell-like，sothatthemoisturecannotexhalethroughtheporousflesh，asitdoesinbirdsandinsnakesandotheranimalswithscalyplates，suchanamountofsecretionisformedthatsomespecialpartisrequiredtoreceiveandholdit。Thisthenisthereasonwhytheseanimals，aloneoftheirkind，haveabladder，thesea-tortoisealargeone，theland-tortoisesanextremelysmallone。