The attachment of the monogenean<i>Discocotyle sagittata</i>Leuckart to the gills of<i>Salmo trutta</i>L.

Abstract

The monogenetic trematode Discocotyle sagittata attaches itself to the gills of Salmo trutta with its long axis inclined to the long axis of the primary lamella to which it is attached, the direction of inclination being determined by the incidence of the gill-ventilating current of the host. The inclination is to some extent brought about by an asymmetrical disposition of the posterior adhesive organs, but Discocotyle appears to retain a primitive unspecialized character in that the asymmetrical disposition is facultative and not obligatory and permanent as in Axine. The adhesive apparatus consists of four pairs of clamps and one pair of hooks. The hooks appear to be the persistent posterior hooks of the larva. Each clamp is operated by an extrinsic muscle-tendon-fair-lead-hinged-jaws system generally similar but possibly less efficient than the corresponding system in Plectanocotyle gurnardi. Scheuring's and Sproston's interpretations of the clamp-closing mechanism of Discocotyle as a spring system are wrong.