OSMOTIC RELATIONS IN MOLLUSCAN CONTRACTILE TISSUES I. ISOLATED VENTRICLE-STRIP PREPARATIONS FROM LAMELLI-

The bivalve molluscs include marine, estuarine and fresh-water forms, and they yield good heart and cilia preparations. Although the ecological distribution and powers of osmoregulation of several species have been studied in detail, little is known about the effects of osmotic pressure on the functioning of their tissues. At the suggestion of Dr G. P. Wells, the writer undertook a comparative investigation of the effects of osmotic pressure variations on the hearts and cilia of lamelli-branchs. The three species used were chosen because they are readily obtained and kept in London, where the work was done, and they represent a wide ecological range. The experiments on hearts are described in the following pages. Those on cilia will form the subject of a second paper. A study of the osmotic pressure relations of rhythmic muscles from three polychaete species was published by Wells & Ledingham (1940). They drew attention to the physiological importance of rate of change of salinity, as distinct from its absolute value. To distinguish between these factors they made experiments of two types, the 'constant exposure', in which the osmotic pressure is suddenly changed to a new value at which it is then held constant, and the 'drift', in which it changes gradually at a controllable rate. The distinction between the actions of osmotic pressure per se and of osmotic pressure change was further emphasized during the course of the present work. MYTILUS EDULIS L. The genus Mytilus is very widely distributed while M. edulis is found on both sides of the North Atlantic. The species occurs over a wide salinity range from full se