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- The characteristic anterior fiber arrangement of the longitudinal muscle layer of adult Neomeniomorpha reveals the developmental fate of the distinct larval (and juvenile) longitudinal muscle units (compare Figs. 3 and 4). The recovery of the same fiber arrangement in both early juveniles and adults clearly argues for muscular transformation by two-dimensional expansion of the originally distinct rectus, enrolling, and ventrolateral muscle bundles into a secondary uniform layer, rather than for reduction and de novo genesis of the longitudinal portion of the adult body wall musculature. Accordingly, the larval rectus, enrolling, and ventrolateral muscles are transformed during metamorphosis and form the innermost layer of the adult body wall musculature. Together with the larval ring and helicoidal musculature, these formerly distinct longitudinal larval muscle bundles form the characteristic adult three-layered body wall musculature. Transitory elements such as the ventromedian muscle and the prototroch muscle ring are lost during neomeniomorph metamorphosis (indicated by “absent”). The number of individual muscles of the inner and outer sets of dorsoventral musculature increases during postmetamorphic body elongation. The larval (and early juvenile) seven-fold seriality of the inner dorsoventral muscles is thus not present in adult Wirenia argentea
