The Splanchnotrophidae is a small family of bizarre poecilostomatoid copepods which utilize marine opisthobranch gastropods, including nudibranchs and pteropods, as hosts. Species have traditionally been placed in this family primarily on the basis of host affiliation, largely neglecting the fundamental differences in morphology and paying virtually no attention to the concept of homology. Morphological analysis based on detailed re-examination of types and newly obtained material from existing museum collections revealed that the Splanchnotrophidae comprises genera drawn from three different families in addition to one non-copepodan taxon. The family Splanchnotrophidae is redefined to include only Splanchnotrophus Hancock and Norman, 1863, Ismaila Bergh, 1867, Lomanoticola Scott and Scott, 1895, and two new monotypic genera. All splanchnotrophids are endoparasites of nudibranch and sacoglossan opisthobranchs and show a vast size disparity between the sexes caused by hypermorphosis in the female. The genus Splanchnotrophus is restricted here to the European species and assumes a boreo-mediterranean distribution. It is redefined on the basis of redescriptions given for S. gracilis Norman and Hancock, 1863, and S. angulatus Hecht, 1893. The Western Australian species S. elysiae Jensen, 1990, and S. sacculatus O'Donoghue, 1924, are re-examined and placed in two new genera, Arthurius and Ceratosomicola, respectively. Re-examination of the mouthparts provided unambiguous evidence justifying formal placement of Briarella Bergh, 1876, in the Philoblennidae, a family thus far known only as ectoparasites from prosobranch gastropods in the Far East. The inadequately described genus Chondrocarpus Bassett-Smith, 1903, is provisionally placed as genus incertae sedis in this family. A new family Micrallectidae is proposed to accommodate Micrallecto Stock, 1971. The genus Nannallecto Stock, 1973, is regarded as a junior subjective synonym of the latter because the generic distinction was largely based on two glaring observational errors: the absence of maxillae in M. uncinata Stock, 1971, caused by imperfect removal of the parasite from the host, and the presence of a chelate leg 2 in N. fusii Stock, 1973, which in reality is a feature of the developing nauplii visible through the body wall of the brooding female. Previous interpretations of the mouthparts in Micrallecto were essentially unsound. Micrallectids are ectoparasites of gymnosome pteropods and display a unique, extremely abbreviated life cycle, involving lecithotrophic nauplii and highly paedomorphic ovoviviparous adults that attain sexual maturity at the metanaupliar stage. Inspection of pteropod collections in the Natural History Museum led to the discovery of the first male specimen providing conclusive evidence for the proposal of a new family. The Micrallectidae is placed in the Poecilostomatoida on the basis of antennary armature, mandibular palp morphology and mating posture. The genus Megallecto Gotto, 1986, is based on a head fragment of a hyperiid amphipod that was erroneously interpreted upside down and back to front. Its type species M. thirioti Gotto, 1986, is identified as a junior subjective synonym of Phrosina semilunata Risso, 1822, a widely distributed and very abundant hyperiid in the Atlantic.
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