The adductor mechanism of pecten

Abstract

The adductor muscles of most lamellibranchs fulfil two functions, first the closing and keeping closed of the valves in response to unfavourable or noxious stimuli, and second occasional partial closure while the animal is respiring and feeding and not strongly stimulated. This last is supposed to be a means of cleaning out the mantle cavity. In Pecten and a few allied forms the adductor mechanism has been modified for swimming by means of rapid rhythmic movements of the valves. The mechanics of the swimming process have been described by Buddenbrock (1). The movements are brought about by the large single adductor muscle (posterior adductor) situated nearly centrally between the valves. Like the adductors of most lamellibranchs the muscle is composed of two easily distinguished parts. The larger part is yellowish and translucent in appearance, of very soft consistency and composed of striated fibres. It is responsible for the rapid flapping movements of swimming. The other part, which generally constitutes less than 10 per cent, of the total weight of muscular tissue, is white and opaque in appearance, much tougher in consistency, and composed of unstriated fibres. It is the part which is responsible for keeping the valves closed, and has been called the “catch” muscle (sperrmuskel, von Uexküll) (2). The two portions of the adductor muscle will be referred to as the large or quick and the small or slow respectively. If one attachment of the slow muscle is severed without other injury to the animal and the quick muscle is stimulated reflexly, by touching the mantle, rapid flapping movements are obtained just as in the intact animal, but the valves cannot be kept in the closed position against the tension of the elastic ligament of the hinge for more than a few seconds. As soon as excitation ceases the valves gape. If, on the other hand, the quick muscle is severed, leaving the slow, the only reaction obtained with the whole animal is a slow closure of the valves which may continue for some time. These phenomena have long been known (Bronn (3), Coutance (4) ). Biedermann (5) in 1885, described what is evidently the twitch of the slow adductor muscle of Anodonta but did not consider it a twitch, apparently because it was several hundred times slower than that of frog’s skeletal muscle (Biedermann (6), vol. 1, pp. 178, 187). Pavlov (7), however, recognised the nature of the response of Anodonta muscle. Prior to the recent work on the mechanical properties of muscle (Gasser and Hill (9), Hill (8), Levin and Wyman (10) ), it was evidently difficult for observers to realise that the same fundamental change might underlie such an enormous difference in time scale, a difference due in part at least to differences in the viscous-elastic constants of the tissue. From the experiments to be described it is concluded that the most important difference between the slow muscle of Pecten and vertebrate skeletal muscle is a matter of “viscosity.”