Abstract
Montacuta tenella Lovén is redescribed, a lectotype for it is figured, and its synonyms are accounted for. The species is presently known to live on the Scandinavian shelf from N. Norway to the Öresund, at moderate depths and in soft bottoms. Evidence is presented indicating that the species lives in association with the spatangid Brissopsis lyrilera. Montacutatenella is a hermaphrodite and its embryos are brooded in the gills of the parent to the veliger stage. Thereafter larval development is pelagic and planktotrophic. Although closely related to M. ferruginosa, M. tenella is remarkable for producing two kinds of sperm in its gonad, the atypical kind being apyrene and vermiform.