The water balance of a serpulid polychaete, Mercierella enigmatica (Fauvel). 3. Accessibility of the extracellular compartment and related studies.

In osmoconforming species fluctuations of salinity in the bathing medium are paralleled by similar variations in the concentration of the body fluids (e.g. Arenicola marina (Schlieper, 1929), Chaetopterus variopedatus (Brown, Bzdil & Frisch, 1972), Mercierella enigmatica (Skaer, 1974a, b)). These animals are found living in media of a wide range of salinity. This implies either that their tissues are in some way protected from changes in the concentration of the body fluids or that their cells survive these changes by means of specializations in their metabolism. The excitable tissues, in particular, are sensitive to changes in their immediate surroundings. The relation between these tissues and their environment has been extensively studied at the cellular level (reviewed by Hodgkin, 1951; Caldwell, 1968; Treherne & Moreton, 1970). Action potentials in these cells are dependent on electrochemical gradients that exist across the cell membranes. These gradients result from differences in the concentrations of inorganic ions between the intracellular and extracellular media. If fluctuations in the ionic concentrations of body fluids were reflected by similar fluctuations in the fluids bathing the excitable cells, then the electrochemical gradients would be upset, at least temporarily, and abnormal action potentials might be expected. The structural relationship between muscle cells and the body fluids is usually simpler than that between nerves and their environment. Nerve cells are often ensheathed in glial and perineurial cells which may form a' blood-brain' barrier (Treherne & Pichon, 1972). This is not the case with muscle cells. The muscles of an osmoconforming serpulid polychaete have been chosen for study. This paper describes the types of muscle cells present and the accessibility of the extracellular fluid surrounding them to the changes in ionic composition that have been shown to occur in the blood (Skaer, 19740,5). Electrical activity recorded from the muscle cells is analysed and correlated with the different types of fibre observed.