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1. Internal microinjection of the Ca‐sensitive photoprotein aequorin or the isotope 45Ca have been used to assess Ca movements in single muscle fibres from the barnacle Balanus nubilusand the crab Maia squinado.

1. The calcium‐sensitive photoprotein aequorin has been used to follow the rapid changes in intracellular calcium concentration that occur during the contraction of single muscle fibres from the barnacle Balanus nubilus, Darwin.

1. In the large single muscle fibres from the barnacle Balanus nubilus, the total fibre Mg concentration was estimated as 15·1 m‐mole/kg wet wt., of which about 3–3·5 m‐mole/kg wet wt. was extracellular. The diffusible Mg, measured by internal sampling, was 11·5 m‐mole/kg wet wt., of which at least half may be complexed to larger diffusible molecules. The free ionized Mg level was estimated as < 5 m‐mole/kg wet wt.

1. The efflux of calcium, as the isotope 45Ca, has been investigated from single muscle fibres from the barnacle Balanus nubilusand from the crab Maia squinado.