FATE OF THE CYPRIS AND ADULT ADDUCTOR MUSCLES DURING METAMORPHOSIS OFBALANUS-AMPHITRITE (CIRRIPEDIA, THORACICA)

The ontogeny of the cypris adductor muscle (CAM) and the adult adducto r muscle (musculus adductor scutorum, MAS) of the acorn barnacle Balan us amphitrite was examined during the cypris/juvenile metamorphosis. B y using section series of epoxy-resin embedded material, we found that the MAS of the adult barnacle develops de novo in a preoral position, while the postoral CAM degenerates but remains present. Eventually th e CAM of the cypris larva disintegrates and the remaining MAS serves t o close the opercular plates of the juvenile and adult barnacle. The C AM and the MAS accordingly do not represent homologous structures and should be encoded as separate characters in a phylogenetic analysis. T he character stares of the CAM and MAS in the taxa comprising the Thec ostraca are reviewed.