Muscles like the smooth adductors of lamellibranch molluscs, and the anterior byssus retractor of
Mytilus
(
ABRM
) possess two remarkable features which have been studied intensively during recent years. They can maintain a steady level of tension (tonic contraction) for longer than any other muscle type (see Marceau 1909), and their contractile apparatus contains, in addition to the usual actomyosin, large quantities of a protein (tropomyosin
A—TMA
) apparently not found in vertebrate striated muscle (Bailey 1956; Kominz, Saad & Laki 1957). As in vertebrate striated muscle, the contractile apparatus of these smooth molluscan muscles contains two kinds of filaments, thin ones and thicker ones (Hanson & Lowy 1959; Philpott, Kahlbrock & Szent-Györgyi 1960). In both muscle types the thin filaments are of similar diameter and contain actin; but the diameter of the thick filaments is about ten times greater in the molluscan muscles, and it is these filaments which contain
TMA
(see Lowy & Hanson 1962). When large quantities of
TMA
were found in these molluscan muscles, their outstanding capacity for tonic contraction was not unnaturally connected with the presence of that protein (Bailey 1957). A hypothesis was proposed according to which there exist in such muscles two independent systems acting in parallel: the usual actomyosin system which produces tension, and a unique system (
TMA
filaments) which, by becoming rigid, can maintain this tension with negligible energy expenditure, after the actomyosin system has relaxed (Rüegg 1958; Johnson, Kahn & Szent-Györgyi 1959). The evidence in support of this ‘independent catch hypothesis’ is based on studies with preparations of
TMA
, on experiments with glycerol-extracted muscles, and on certain experiments with the living
ABRM
treated with thiourea. Results obtained from structural studies and from extensive investigations on living tonic molluscan muscles have suggested an alternative hypothesis, the ‘linkage hypothesis’. This holds that, as in vertebrate striated muscle, tension in tonic molluscan muscles is both generated and maintained by the same system, which is one that involves interaction between adjacent actin- and myosin-containing filaments in a sliding filament system (Lowy & Millman 1959
b
, 1963).