Additions to the Morphology of the Cucumariidae. I. Intraspecific Variation of the Retractor Muscle in Parathyone braziliensis (Verrill, 1868) and Thyonidium seguroensis (Deichmann, 1930) (Holothuroidea: Dendrochirotida)

Intraspecific variation of the gross morphology of the retractor muscle was studied in 60 specimens of Parathyone braziliensis and 17 specimens of Thyonidium seguroensis . Specimens were dissected to expose the retractor muscle or were analyzed by micro-computed tomography. Early expectations that the point of origin of the retractor muscle at the longitudinal muscle relative to the midpoint of the body length was age-related and that the split of the retractor muscle into separate muscle bands could be used as taxonomic characters to separate species were not supported by anatomical evidence.