Features of the spermatozoon ofCalocarcinus africanuswhich are general for heterotremes and endorse its inclusion in the Heterotremata are: extension of the subacrosomal chamber almost to the anterior apex of the sperm, presence of an acrosome ray zone, and presence of a thickened ring where the capsule surrounds the base of the subacrosomal chamber. A feature shared with “higher” heterotremes is the restriction of cytoplasm to the periacrosomal region, the arms being nuclear only, in contrast with invasion of their chromatin with cytoplasm and microtubules in majids; and loss of a posterior median process, containing chromatin, which is present in majids as in raninids and homolids. The relationship ofCalocarcinuswith xanthids is unequivocally supported by (1) presence of a posterior circumperforatorial zone, the xanthid ring; (2) the precise form of the acrosome ray zone which is wide anteriorly and sends a long slender extension posteriorly to meet the ring; (3) the irregular external margin of the outer acrosome zone; (4) division of the operculum complex into a distinct upper zone and a lower, subopercular zone of lesser diameter; and (5) presence of an accessory ring around the operculum. Modification of the xanthid ring is typical of thoracotremes and partial modification inCalocarcinusmay indicate origin of thoracotremes from a related xanthoid stock. A difference ofCalocarcinussperm from those of xanthids is the (plesiomorphic) presence of centrioles, also seen in some heterotremes and thoracotremes. No synapomorphies which are not common to other heterotremes are shared betweenCalocarcinusand trapeziid sperm. Trapeziid sperm (plesiomorphically?) lack the xanthid ring, the posterior exteasion of the acrosome ray zone and the irregular margin of the outer acrosome zone ofCalocarcinusand xanthids. Apomorphic features of theCalocarcinus africanussperm include a spiral configuration of the contents of the outer acrosome zone (autapomorphy?), as seen in cross-section, and presence of a periopercular rim. A well developed periopercular rim is known elsewhere only inPotamonautes(family Potamidae), but a rudiment occurs in some xanthids (e.g.,Etisus). The periopercular rim is probably a true synapomorphy indicative of relationship of potamids to xanthoids (represented byCalocarcinus) which has been postulated elsewhere on morphological grounds.