The attachment of the monogeneanDiplectanum aequans(Wagener) Diesing to the gills ofMorone labraxL.

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Abstract
The main adhesive apparatus ofDiplectanum aequansis described and a functional interpretation is offered.Two sets of organs are involved in adult animals: 4 hamuli (median hooks) and the 3 associated accessory bars, and 2 squamodiscs.The two pairs of hamuli are ‘gaffed’ into the gill tissue of the host by the following mechanism: muscular contraction produces a posterior movement of the anterior ends of the hamuli which are so pivoted that the points of the hamuli are caused to extend out from the haptor.The squamodiscs of adults are operated by extrinsic muscles which produce posterior movement in the anterior region of each squamodisc. This tends to push the parasite out from its position between adjacent secondary gill lamellae.The tendency to dislodgement is opposed by the two pairs of hamuli which ‘gaff’ into the base of the secondary lamellae. In addition, the more posterior spines on the squamodiscs become embedded in the gill tissue and so provide further resistance to the movement of the anterior region of the squamodisc. Thus the attachment of adults requires both sets of adhesive organs functioning in a complementary manner.Juveniles do not possess squamodiscs. They are considered to adhere to their hosts primarily with their hamuli.I would like to acknowledge the help given to me by the Director and Staff of the Marine Biological Association Laboratory and also the Proprietors of Cook and Sons Ltd., of Salcombe, Devon.