The teleost stomach, as a site of strong acid proteolytic digestion at pH 1–4, is a hostile environment rarely chosen as a habitat by parasites. Several Hemiuroidea have nevertheless invaded this niche, notably the Hemiuridae, a family in which the body is characterized by a retractile “tail”, or ecsoma. The structure and function of the latter has been studied in three hemiurid species, namely Hemiurus communis, Lecithochirium rufoviride and L. fusiforme, using ultrastructural, histochemical and autoradiographic techniques. Results indicate a clear demarcation between the function of the tegument covering the ecsoma and that of the rest of the body, or soma. No mitochondria have been observed in the ecsomal tegument, and intense acid phosphatase activity was detected in association with its outer plasma membrane. Uptake of 3H-tyrosine occurred within the excretory vesicle, which may serve for transport of nutrients forwards into the soma. In contrast, the thickened, dense somal tegument, with few mitochondria largely confined to ridges or plications in H. communis, showed neither phosphatase activity nor uptake of tyrosine. Glucose uptake was not detected in any part of the body. That the somal tegument serves not for absorption but for protection against both physico-chemical conditions and mechanical damage by prey entering the host's stomach is discussed.

Ultrastructural, histochemical and autoradiographical techniques have been used to investigate the development and function of the tegument of both somal and ecsomal body regions in the hemiurid Lecithochirum furcolabiatum. The terms mesocercaria and metacercaria are here adopted for those stages in the copepod second and fish third intermediate hosts respectively on the basis of morphology and on analogy with the Strigeidae. Mesocercariae were obtained by experimental infection of the harpacticoid copepod Tigriopus brevicornis with the cystophorous cercariae (syn. Cercaria vaullegeardi), whilst metacercariae were removed from the body cavities of naturally-infected rockpool teleosts including Gobius paganellus and Blennius pholis. Observations on the mesocercaria show the origin of the ecsoma from the distal half of the excretory vesicle, which at 21 days post-infection is capable of eversion through the terminal pore. The nucleated microrugous surface layer of the ecsoma at this stage is modified or replaced in the metacercaria by anucleate syncytial tegument similar to that of the adult organ. The metacercarial stage is also associated with the more advanced development of the somal tegument. The two stages differ in metabolic activity, the somal tegument of only the metacercaria resembling that of the adult in acid phosphatase distribution. In the mesocercaria the ecsomal tegument showed the strongest uptake of 3H-tyrosine; absorption of the latter was not detected in the metacercaria. 3H-glucose uptake was restricted to the intestinal caeca of both stages.

Robinia aurata n. g., n. sp. is described from Liza aurata (Mugilidae), the golden grey mullet, from the Ebro Delta, Spanish Mediterranean. The new genus differs from all other hemiurid genera in the combined possession of muscular flanges and a vestigial ecsoma. Within the Bunocotylinae, which currently accommodates 2 genera, Bunocotyle and Saturnius, the new genus exhibits a unique combination of blind caeca, Juel's organ, post-ovarian bulk of the uterus in the hind-body, and tegumental papillae surrounding the oral and ventral sucker apertures. Furthermore, Robinia n. g. differs from both Bunocotyle and Saturnius in the nature of the muscular extensions around the oral sucker, with the shape of a muscular belt in the latter and numerous muscular papillae in the former. The phylogenetic hypothesis for the Bunocotylinae developed from sequence data analyses based on partial lsrDNA and complete ssrDNA combined (22 species) and V4 domain of the ssrRNA gene (37 species) supports the erection of the new genus and confirms its position within the Hemiuroidea. Both molecular analyses confirmed the monophyly of the Hemiuroidea, its division into 2 major clades and the polyphyly of the Derogenidae, as in previous studies, and suggest that the Gonocercinae (with 2 genera, Gonocerca and Hemipera), may require a distinct familial status. Finally, there was poor support for the distinct status of the Lecithasteridae and Hemiuridae, following previous suggestions based on different sequence data sets. A key to genera of the Bunocotylinae is presented.