On the Ciliary Mechanisms and Interrelationships of Lamellibranches

Abstract

In the gill axes of the Microciliobranchia the most important muscles are longitudinal and transverse. The longitudinal muscles are: (a) those extending from one extremity of the gill axis to the other, inserted on the shell anteriorly, and (b) those in the free posterior portion of the axis, inserted on the shell where the axis becomes attached. Together these muscles act as branchial retractors. Withdrawal of the gills prevents (a) their being caught and crushed by the edges of the shell when the valves are suddenly closed, and (b) excessive fouling with sudden intake of muddy or noxious water. The transverse muscles below the chitinous structure arching the axial food groove serve to draw the demibranchs of a gill together, while those above the arch serve to separate them. Such swaying movements of the demibranchs serve to rid them of unwanted material. In the demibranchs are:--(1) muscles of the free edges. These include (a) muscles responsible for movements of the walls of the food grooves, and (b) longitudinal muscles, which effect antero-posterior contraction and assist the longitudinal muscles of the axis in retraction of the gills; (2) vertical muscles of the demibranchs, found chiefly in the Pteriacea, and responsible for dorso-ventral contraction of the demibranchs; (3) muscles of the interlamellar junctions serving to draw the two lamellae of a demibranch together, expelling the contained water; (4) horizontal muscles of the lamellae, present in forms with plicate and heterorhabdic gills and effecting by their action changes in shape of the frontal surface of the principal filaments and movements of the plicae important in connexion with the ciliary sorting mechanism; their contraction increases the folding of the lamellae and decreases the length of the gill: and (5) fine muscle-fibres forming the intrafilamentar ‘septum’.