First description of an apical epidermis cone in Paratenuisentis ambiguus (Acanthocephala: Eoacanthocephala) and its phylogenetic implications.

The proboscis apex of the eoacanthocephalan parasite Paratenuisentis ambiguus was studied, using electron and light microscopy, for a better understanding of the parasite's attachment at the intestinal wall of its definitive hosts (Anguilla rostrata and A. anguilla). The results suggest the presence of an epidermis cone with three nuclei at the proboscis apex of P. ambiguus instead of an apical sense organ, as has been previously supposed. Dendritic terminations, sensory nerves and secretory ducts were absent. The existence of many fibres suggests a mechanical function of the epidermis cone. Probably, it presses the proboscis apex and the anterior hooks into the intestinal wall of the definitive host. The presence of an epidermis cone in other eoacanthocephalan species can be derived from data in the literature. The absence of an epidermis cone outside the Eoacanthocephala suggests that it is an evolutionary innovation, supporting the monophyly of the Eoacanthocephala.