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Inrecentyears,manycryptictaxahavebeendiscoveredamongboth
theMonogonontaandtheBdelloidea.Cryptictaxawereearlierconsideredasone
speciesbutmtDNAstudieshaverevealedtheirdiversificationintomanyspecies
(GOmEZandSNELL1996;GOmEZetal.2002;DERRYetal.2003;GOmEZ2005;
FONTANETOetal.2008;FONTANETOetal.2011).Theresultsofthesestudiesindicate
thatthediversityofRotiferaissignificantlygreaterthanhasbeenacceptedsofar
(FONTANETO2014).
3.Taxonomyandsystematics
Asinmanyotherinvertebrategroups,rotifertaxonomy,systematicsandclas-
sificationpresentahostofextremelydifficultproblems,whicharereflectedby
continuousmodificationsoftherotifersystem.Duetotheincompletelyexplained
originsofrotifer,theirunparalleledmorphologicalandanatomicalvariations,
ahighintraspecificvariability,andaparticularlycomplexinternalstructure,
taxonomicandsystematicdivisionsofrotifershaveoftenproduceddoubtsand
ambiguities.Theprogressingeneticstudiesonrotiferpopulationscastsanew
lightonrotiferoriginsandfundamentalsoftheirtaxonomicclassification(KOSTE
andSHIEL,1989;HILLISandmORITZ,1990)andhasledtostillnewmodifications
oftherotifertaxonomicsystemswhichwerebasedprimarilyonclassicmorpho-
logicalcharacteristics.Thus,thehistoryofrotiferstudies,morethanonehundred
yearsold,aboundsintaxonomicsystems,butonlyfewofthemhaveprovento
beofakeyimportanceforthedevelopmentofcontemporaryanimalsystematics.
TheoldestsystemofrotifertaxonomyisthatdevelopedbyEHRENBERG(1838)
whodividedtheclassRotiferainto2largegroups,theMonotrochaandtheSoro-
trocha,eachcontaining4familiesdifferinginthestructureoftheirwheelappa-
ratus.Inaddition,Ehrenbergdescribedtheanatomyofnumerousrotiferorgans.
InEhrenberg’ssystem,somecloselyrelatedspeciesareveryfrequentlylocated
farapart.
Subsequently,WESENBERG-LUND(1899)proposedtousemorphologicaland
anatomicalcriteriatodividetheclassintothefollowingtwogroups:theDi-
gononta(withpairedgonads)andtheMonogononta(withunpairedgonads).
TheDigonontaweresubdividedinto2orders:theBdelloidea(with2gonadsand
thevitellarium)andtheSeisonacea(with2gonads,withoutthevitellarium),while
theMonogonontaweresubdividedintothreeorders:thePloima,Flosculariacea,
andCollothecacea.
DEBEAUCHAmP(1965)workedoutarotifersystembasedondifferencesin
thewheelapparatus.HesubdividedtheMonogonontaintotwoorders:thePseu-
dotrochaandtheGnesiotrocha,thelatterbeingfurtherdividedintotwosub-or-
ders:theMonimotrochaandthePaedotrocha.Thesub-ordersdiffermarkedlyin
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