The accessibility of the coelomic and extracellular fluid compartments to the blood and the external bathing medium have been studied in the serpulid polychaete Mercierella enigmatica. Only the single cell layer of the blood-vessel walls separates the blood from the other body fluids. It is likely that this layer is relatively permeable to water and to molecules of low molecular weight so that changes in ion concentration at the surface of tissues, such as the muscles, probably parallel the

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... nsive changes in the blood. Two types of longitudinal muscle fibre, differing in filament diameter, cell shape and distribution have been observed. The pattern of electrical activity recorded by intracellular micro-electrodes falls into two classes, separable on the basis of several parameters. This pattern is discussed in relation to the two types of longitudinal muscle fibre found.