Parasitic ‘turbellarians’ are known from various animals such as echinoderms, crustaceans, annelids, bivalve and gastropod molluscs. So far, however, no ‘turbellarians’ have been reported from cephalopods. In this paper we report a parasitic ‘turbellarian’ from the giant Antarctic octopus, Megaleledone setebos. We dissected two specimens of M. setebos caught in the Ross Sea (Antarctica) and found numerous worms in their intestine and liver. The worms were spherical or oblong and had two morphologically different poles. The frontal pole bears a small conical protrusion containing large elongated pear-shaped frontal glands and large polygonal cells. The ducts of the frontal glands open terminally to form the frontal organ. The caudal pole has an opening shaped as a folded tube connected by the genital pore with a common genital atrium, which continues into a canal with a muscular sheath. The worms were identified as ‘turbellarians’ from the family Notenteridae (Fecampiida). This family contains only one species, Notentera ivanovi, reported from the gut of a polychaete at the White Sea. The worms that we found in the gastrointestinal tract of the octopuses were morphologically similar to N. ivanovi but differed from it in several important respects. Phylogenetic analysis based on 28S rDNA gene showed that the newly found worm clustered together with other fecampiids in a highly supported clade and was closely related to N. ivanovi. On the basis of these morphological and molecular data, we described a new species, Octopoxenus antarcticus gen. nov., sp. nov. (Fecampiida: Notenteridae), establishing a new genus to accommodate it and provided an updated diagnosis of the family Notenteridae. This is the first report of a parasitic ‘turbellarian’ from a cephalopod mollusc.

A new species, Philosyrtis aegusae sp. nov., collected in the ‘Otoplanen-Zone’ of the sands of western Sicily, is described. The specimens show the typical morphological peculiarities of the subfamily Parotoplaninae (‘Turbellaria’: Otoplanidae), but differ clearly from the species already described. The body length of the sexually mature animals reaches 0.9–1.1 mm and they appear to belong to the genus Philosyrtis because of the general arrangement of testes and germo-vitellaria, and of the sclerotic apparatus. The vesicula granulorum is clearly distinct from the sclerotic apparatus unlike the other ten species belonging to the same genus. The male copulatory organ consists of a dorsal group of 7–8 spines similar in shape and length (33–40 mm), and a ventral group of at least 6 long thin spines with curved pointed tips. The new species differs from the already described species above all in its body dimensions, the shorter path of the vitellaries and especially the characteristics of the male sclerotic apparatus.

The morphology and taxonomy of a new species of otoplanid (Plathelminthes: Rhabditophora: Proseriata) is discussed. Otoplana proxima sp. nov., collected at Marina di Bibbona (Livorno), presents the typical morphological peculiarities of the subfamily Otoplaninae, but clearly differs from the previously described species with regard to the organization of the genital organs. The new flatworm appears more similar to Otoplana intermedia, but differs sharply in its body length and male copulatory organ made up of 23–24 pliable spines (42–70 μm long) characterized by a forked tip and a canaliculated proximal end.

Two new species of polyclads are described from the continental slope of the Gulf of Mexico. Specimens of Didangia carneyi sp. nov. and Oligocladus bathymodiensis sp. nov. were collected from the Louisiana slope at 610 m and 650 m, respectively. Didangia carneyi sp. nov. was collected from a natural wood fall, and is characterized by the presence of tentacular and cerebral eyes, an interpolated prostatic vesicle provided with two muscular accessory prostatic vesicles, and large glandular cells that surround the male atrium. Oligocladus bathymodiensis sp. nov. has a mouth anterior to the brain, a few cerebral and pseudotentacular eyes, four pairs of intestinal branches, and a ventral anal pore. Specimens of this species were collected on the margin of a hypersaline cold seep in association with mussels of Bathymodiolus childressi. All type material is deposited at the Field Museum of Natural History, Chicago, Illinois, USA.

The flatworm nervous system employs a wide repertoire of neuroactive substances, including small chemical messengers, the so called classical transmitters, and several types of neuropeptides. A large body of research accumulated over four decades has provided a wealth of information on the tissue localization and effects of these substances, their biochemistry and, recently, their molecular modes of action in all major classes of flatworms. This evidence will be reviewed, with particular emphasis on the small (classical) transmitters and the receptors that mediate their effects. One of the themes that will emerge from this discussion is that classical transmitters regulate core activities such as movement, metabolism and transport, and thus are essential for survival of the organism. In addition, the evidence shows that flatworms have multiple neurotransmitter receptors, many with unusual pharmacological features, which make them particularly attractive as drug targets. Understanding the molecular basis of these distinctive properties, and developing new, more specific receptor agonists and antagonists will undoubtedly become a major challenge in future research.