The attachment of the monogeneanDiscocotyle sagittataLeuckart to the gills ofSalmo truttaL.

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Abstract
The monogenetic trematodeDiscocotyle sagittataattaches itself to the gills ofSalmo truttawith 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, butDiscocotyleappears to retain a primitive unspecialized character in that the asymmetrical disposition is facultative and not obligatory and permanent as inAxine.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 inPlectanocotyle gurnardi.Scheuring's and Sproston's interpretations of the clamp-closing mechanism ofDiscocotyleas a spring system are wrong.