MICROMORPHOLOGY AND GENE EXPRESSION IN MUSCLE AND SHELL DEVELOPMENT OF THE MOLLUSCA
b'This work comprises detailed studies by scanning electron microscopy\\n(SEM), transmission electron microscopy (TEM), fluorescence staining combined with\\nconfocal laser scanning microscopy (CLSM), as well as serial sectioning analyses and\\nreconstruction techniques to elucidate the development of the larval and adult musculature of\\nseveral basal representatives of the molluscan classes Polyplacophora, Bivalvia, Scaphopoda,\\nand Gastropoda. Special reference is given to the shell musculature. In addition, aspects of the\\nmyo-anatomy of adult Solenogastres are reconsidered. A further part of this study deals with\\nscaphopod shell morphogenesis and expression of the homeobox gene engrailed (en), in order\\nto gain insights regarding the scaphopod-bivalve relationship. The results enable far reaching\\nconclusions regarding the evolution and the phylogeny of the Mollusca.\\nSolenogastres\\nTEM analysis of adult Solenogastres revealed a mesenchymate body wall musculature\\nwhich consists of outer ring, intermediate diagonal, and inner longitudinal muscles and\\nresembles the condition of other worm-shaped taxa. The ventrally inter-crossing dorso-ventral\\nmusculature, which is diagnostic for the Mollusca, is arranged in multiple serial units along\\nthe anterior-posterior body axis.\\nPolyplacophora\\nDuring development, the chiton larva undergoes an intermediate stage in which the\\ndorso-ventral musculature is serially arranged as in adult Solenogastres. The concentration\\ninto seven (and later eight) functional shell plate muscle units is a secondary condition which\\ntakes place after metamorphosis. Thus, assumptions of a primarily "segmented" (i.e. annelidlike)\\ncharacter of the polyplacophoran shell plate musculature are rejected. In addition, the\\nanterior (i.e. pre-trochal) body region of chiton larvae shows a muscular grid which is lost at\\nmetamorphosis and resembles the body wall musculature of adult aplacophoran\\n(Solenogastres + Caudofoveata) molluscs. Both, the multiple seriality of the dorso-ventral\\nmuscles and the apical muscle grid are regarded as ontogenetic recapitulation of the basal\\nmolluscan condition which is fully expressed in the adult body plan of Solenogastres. This\\ninfers a non-segmented, worm-shaped ancestor at the base of molluscan evolution.\\nThe existence of a larval ring-shaped muscle that underlies the prototroch cells\\n(prototroch muscle ring) is a shared feature of polyplacophoran, gastropod, and bivalve larvae\\n(see below) and suprataxic homology of this organ is proposed.\\nBivalvia\\nBesides a rather complicated set of larval retractor muscles, the veligers of\\nautobranchs (i.e. all Bivalvia except the Protobranchia, the latter with a test-cell larva) exhibit\\na distinct prototroch muscle ring similar to chitons and gastropods. Both systems are entirely\\nlarval and are resorbed during metamorphosis.\\nScaphopoda\\nThe general ontogeny and especially myogenesis in the dentaliid scaphopod Antalis\\nentalis proceeds much more direct than in polyplacophorans or gastropods. Accordingly,\\ndistinct larval muscle systems are lacking. However, the paired cephalic and pedal retractors\\nboth form additional fibers which project into the region of the prototroch and are lost at\\nmetamorphosis. The existence of a distinct, paired cephalic retractor system, which is also\\nfound in the basal gastropod and cephalopod bauplan but not in the Bivalvia, suggests a clade\\ncomprising the Scaphopoda and Gastropoda + Cephalopoda. This is strengthened by\\nexpression data of the homeobox gene engrailed, which plays a significant role in molluscan\\nshell formation. While two dorso-lateral centers of engrailed expression, which correspond to\\nthe two centers of initial shell calcification, are found in early bivalve veligers, engrailed is\\nexclusively found in mantle margin cells surrounding the single anlage of the embryonic\\nscaphopod shell. In contrast to bivalves, the scaphopod shell is thus formed from a single\\ncenter of calcification, and a scaphopod-bivalve sistergroup relationship is therefore rejected.\\nGastropoda\\nPrimitive gastropods, such as the patellogastropods Patella vulgata and Patella\\ncaerulea, show one pair of asymmetrically positioned larval retractor muscles which have\\ndistinct insertion sites at the embryonic shell. Another strict larval muscle system is the\\nprototrochal muscle ring. All these muscle are lost before, during, or shortly after\\nmetamorphosis. Parts of the adult mantle musculature as well as the muscles of the cephalic\\ntentacles are formed prior to metamorphosis, while the buccal musculature is of entire postmetamorphic\\norigin.\\nThe process of gastropod ontogenetic torsion is mainly caused by muscular activity of\\nthe larval retractors, while the adult shell musculature arises after the completion of torsion.\\nThus, ontogenetic torsion is regarded as an entirely larval process inferring that the\\narrangement of the adult shell musculature - which can often be reconstructed by muscle scars on fossilized shells - is not indicative for the question whether paleozoic univalved molluscs\\nwere torted or not.'