thereby , reconstruction of the last common ancestor ( lca ) to all mollusks , the so - called hypothetical ancestral mollusk , has been hampered by difficulties in recovering fossils of unambiguous molluscan stem species and by the lack of a general agreement concerning the relationships of the various molluscan class - level taxa to each other . as a consequence ,
morphology - based analyses have suggested either one or the other of the sclerite - bearing but shell - less aplacophoran clades ( neomeniomorpha or chaetodermomorpha , respectively ) as the earliest molluscan offshoot [ 8 , 9 ] , a monophyletic aplacophora as sister group to all remaining mollusks ( the testaria ) , or a polyplacophoran - aplacophoran assemblage ( aculifera ) as sister to all other mollusks with a primarily univalved shell ( conchifera ) [ 11 , 12 ] .
this controversy may soon be settled , however , since two phylogenomic studies have independently confirmed the latter concept , whereby both recovered a monophyletic aplacophora as sister group to polyplacophora ( chitons ) within aculifera [ 4 , 5 ] . in the light of this phylogenetic framework ,
the recent description of a cylindrical ( i.e. , worm - shaped ) sclerite- and shell - bearing paleozoic mollusk   and the results of integrative molecular - paleontological studies [ 13 , 14 ] have been considered as evidence for the presence of seven or eight shell plates in the lca of crown - group aculiferans . if correct , this implies that the body plan of recent aplacophorans is the result of secondary simplification and thus a derived condition [ 6 , 14 ] . despite additional descriptions of fossils that exhibit a mixture of polyplacophoran- and aplacophoran - like features [ 15 , 16 ]
this may be due to the overall paucity of well - preserved paleozoic fossils that undoubtedly can be assigned to the aculiferan lineage , the uncertainty as to whether or not some early ( cambrian or precambrian ) fossils [ 17 , 18 ] indeed represent crown- or stem - group mollusks , and the fact that solid morphological and developmental evidence from recent aplacophoran representatives is still largely lacking
. the few reports of individual aplacophoran larvae and postlarvae have shown that these animals may bear six or seven rows of papillae , sclerites , or sclerite - secreting cells [ 1921 ] , but these studies found no further support by gross morphological developmental studies of two neomeniomorph representatives [ 22 , 23 ] .
the muscular architecture of mollusks is intimately associated with the existence , number , and arrangement of shells in the respective taxa . in polyplacophorans ,
the myoanatomy is highly complex [ 7 , 25 ] ( figures 1a and 1c ) and several components , such as a laterally positioned enrolling muscle and a dorsal rectus system that spans the longitudinal axis of the animal , have been widely considered as defining morphological features ( autapomorphies ) of this taxon . in stark contrast to the sophisticated polyplacophoran myoanatomy , aplacophoran representatives have a much simpler muscular organization that , together with the body wall musculature , mainly comprises serially repeated dorsoventral muscles ( figures 1b and 1d ) .
accordingly , a scenario that suggests a shell plate - bearing aculiferan lca with polyplacophoran - like musculature implies drastic secondary simplification of the muscular body plan of aplacophoran mollusks . since ontogenetic data may provide important insights into the evolutionary history of a given taxon [ 26 , 27 ] , we investigated the development of a model neomeniomorph aplacophoran , wirenia argentea , from hatching of the larvae until after metamorphosis . in comparing myogenesis in wirenia with that of a polyplacophoran ( leptochiton asellus )
, we found striking similarities in the muscular organization of both species ( see table 1 for a summary of major larval and adult muscle systems known for the various molluscan lineages ) , including the presence of a rectus muscle ( figures 2a2d ) , so far only known from polyplacophorans , and a laterally positioned enrolling muscle ( figures 2a2h ) .
both systems are retained in adult polyplacophorans but are lost during wirenia postlarval development ( note that some neomeniomorphs do exhibit distinct enrolling muscles as adults [ 28 , 29 ] ) . although homology between the polyplacophoran and neomeniomorph enrolling muscles has been questioned , their similar position in the respective animals and , in particular , their identical mode of ontogenetic formation as independent muscle system ( and not as a thickened derivative of the body wall musculature , as proposed earlier for the aplacophoran taxa ) argue strongly for their common evolutionary origin and thus for the presence of such a system in the lca of both clades . aside from these muscles
both systems are only transiently present in advanced larvae , and neither has a counterpart in either the polyplacophoran or the neomeniomorph adult body plan . as with the rectus and the enrolling muscles , the identical positions of the ventrolateral and the ventromedian muscles in wirenia and leptochiton larvae , together with their identical positions relative to other muscles , clearly suggest that these respective muscles are homologous between the two species ( see   for a recent overview on homology theory and assessment ) . earlier , we had already found the ventrolateral system in larvae of another polyplacophoran , mopalia muscosa , but had overlooked the ventromedian muscle .
reinvestigation of the original 3d data set , however , unambiguously revealed such a muscle also in mopalia ( data not shown ; available on request ) .
accordingly , it appears highly likely that the ventrolateral and the ventromedian muscles together with the enrolling and the rectus muscle were all part of the muscular toolkit of the lca of polyplacophorans and neomeniomorphs , and that the simple myoanatomy of adult wirenia is a derived , secondary condition .
formation of the eight sets of dorsoventral muscles in polyplacophorans passes through a transitory stage of multiple individual myocytes that appear synchronously   ( figure  2e ) and give rise to the first seven paired shell muscle units ( while the eighth set forms considerably later , together with the most posterior shell plate ) .
in wirenia , seven pairs of dorsoventral muscles develop simultaneously ( figures 3a3e ) and differentiate further in later stages ( figure  2f ) .
a gradual numerical increase of the dorsoventral muscle sets was observed only after metamorphosis ( figure  3f ) .
accordingly , both neomeniomorphs and polyplacophorans exhibit a transient stage of seven - fold seriality in the arrangement of these muscles . despite the different ontogenetic pathways that lead to this seven - fold seriality ( fusion of multiple myocytes in polyplacophorans versus simultaneous formation in wirenia )
, this seven - fold seriality appears to be a reoccurring pattern , at least in aculiferan mollusks ( unfortunately , the ontogenetic sequence of the formation of the eight pairs of dorsoventral muscles in monoplacophora is still unknown ) .
this is well in line with the description of the seven - shelled fossil kulindroplax , as well as with the serially arranged sclerites or papillae of some recent aplacophorans [ 1921 ] , and lends further support for an aculiferan lca with a seven - fold seriality of epidermal hardparts and the associated musculature .
this implies that the eighth set of dorsoventral muscles of recent polyplacophorans is a derived condition .
the late formation of the most posterior shell plate and associated musculature in polyplacophorans [ 7 , 31 ] may well be considered as ontogenetic testimony of such a scenario .
the rudimentary data on myogenesis ( and development in  general )   of the second aplacophoran taxon , the chaetodermomorpha , does not allow for definite conclusions concerning the presence of neomeniomorph- and/or polyplacophoran - like features such as the rectus or the ventromedian muscles in the lca of this taxon .
however , the ring musculature in the body wall of both aplacophoran clades , which is also rudimentarily present in the apical region of polyplacophoran larvae   ( figure  2 ) , may constitute an aculiferan apomorphy uniting polyplacophora , neomeniomorpha , and chaetodermomorpha ( table 1 ) .
this , together with the cylindrical shape of the aplacophorans and the polyplacophoran larva as well as the fossil kulindroplax , argues for a worm - like body shape of the lca of aculifera , rendering the dorsoventrally flattened appearance of recent polyplacophorans a derived condition .
the paired lateral longitudinal muscle in the larva of the chaetodermomorph chaetoderma   may correspond to either the enrolling or the ventrolateral muscle of larval polyplacophorans and neomeniomorphs ( table 1 ) , which would further support the inclusion of chaetodermomorpha within aculifera .
the fact that kulindroplax shares morphological features not only with polyplacophorans but also with recent chaetodermomorphs , including the absence of a pedal pit and the position of the gills , likewise supports such a scenario .
whether or not a monophyletic aculifera will stand the test of future phylogenetic analyses or whether chaetodermomorpha , despite these shared morphological characters , will be proven to have different affinities   remains to be seen .
the transient expression of typical polyplacophoran - like muscles in the wirenia larva , however , strongly suggests that at least neomeniomorph aplacophorans stem from an ancestor with polyplacophoran - like features that most likely also included seven shell plates .
 summarymollusca is an animal phylum with vast morphological diversity and includes worm - shaped aplacophorans , snails , bivalves , and the complex cephalopods [ 1 ] . 
 the interrelationships of these class - level taxa are still contentious [ 2 , 3 ] , but recent phylogenomic analyses suggest a dichotomy at the base of mollusca , resulting in a monophyletic aculifera ( comprising the shell - less , sclerite - bearing aplacophorans and the eight - shelled polyplacophorans ) and conchifera ( all other , primarily univalved groups ) [ 4 , 5 ] . the aculifera concept has recently gained support via description of the fossil kulindroplax , which shows both aplacophoran- and polyplacophoran - like features and suggests that the aplacophorans originated from a shelled ancestor [ 6 ] , but the overall morphology of the last common aculiferan ancestor remains obscure . here 
 we show that larvae of the aplacophoran wirenia argentea have several sets of muscles previously known only from polyplacophoran mollusks . 
 most of these are lost during metamorphosis , and we interpret them as ontogenetic remnants of an ancestor with a complex , polyplacophoran - like musculature . 
 moreover , we find that the first seven pairs of dorsoventral muscles develop synchronously in wirenia , similar to juvenile polyplacophorans [ 7 ] , which supports the conclusions based on the seven - shelled kulindroplax . 
 accordingly , we argue that the simple body plan of recent aplacophorans is the result of simplification and does not represent a basal molluscan condition . 