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)

Abstract

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.