Nauplii, copepodids and adults of a new mesoparasitic genus and species of Chitonophilidae, Lepetellicola brescianii, are described from the pallial cavity of a deepwater cocculiniform limpet, Lepetella sierrai, collected in the Bay of Biscay and Gulf of Cádiz. Re-examination of the type material of the recently established Nucellicolidae revealed several important observational errors in the original description, such as the oversight of the rootlet system in the adult female and misinterpretations of the tagmosis and antennulary segmentation in the late copepodid. Lamb et al.’s (1996) criteria used to justify the familial distinctiveness of the Nucellicolidae are all invalid. The family is relegated to a junior synonym of the Chitonophilidae on the basis of overwhelming support provided by copepodid and adult morphology. The impact of heterochrony on the body plan of adults and developmental stages is discussed. Phylogenetic analysis supports a basal dichotomy dividing the Chitonophilidae into a mesoparasitic clade, utilizing exclusively polyplacophoran hosts, and a sisterclade grouping genera associated with chitons, prosobranch gastropods and cocculiniform limpets. The presence of maxillipeds and postmaxillipedal apodemes in the adult males of the latter clade is considered as apomorphic rather than plesiomorphic, being the result of incomplete moulting and correlated with the ventral position of the genital apertures. Nucellicola is identified as the sistergroup of the only other endoparasitic genus, Tesonesma, found in the body cavity of chitons. The inferred relationships indicate that host switching has occurred twice in the evolution of the Chitonophilidae.Examination of the antennulary segmentation and setation patterns of copepodids in Lepetellicola and Nucellicola unequivocally refutes both the current placement of the Chitonophilidae in the Poecilostomatoida and its alternative assignment to the Siphonostomatoida. Exclusion from the Poecilostomatoida is reinforced by the absence of a coxo-basis in the antenna. The family is placed with the cyclopoids, providing further evidence for the crown-group status of the Poecilostomatoida within the currently paraphyletic Cyclopoida. A critical review of the published reports of brooding in cocculiniform limpets demonstrated that there is, as yet, no tangible evidence for this phenomenon in either the Lepetelloidea or Cocculinoidea.