In this paper some of the work on large nerve and muscle fibres from marine animals done in recent years at the Plymouth Laboratory of the Marine Biological Association of the United Kingdom is discussed. The main piece of work has been concerned with the question of the active transport of cations in the giant axons of the squid, Loligo/orbesi. These giant axons are usually 500--1000 # in diameter and can be obtained in lengths of several centimetres. It is possible to cannulate these axons after dissection and to insert various types of electrode or micro-injector into them (HoDGI~I~ and HuxLeY 1939, 1945; HODGKI~ and KEY~ES 1956). In the course of this work the micro-injector designed by HODGKIN and K~Y~-Es (1956) for use with squid giant axons has been used extensively and the experimental arrangement employed in the work on the effects of injected substances on the efflux of the isotope Na ~2 from squid axons is shown in Fig. 1.
In 1955 HODGKI~" and K E Y ~s published experiments which showed that the Na e4 efflux and K 42 influx of Sepia giant axons and the Na 24 efflux of squid giant axons were greatly reduced in axons poisoned by inhibitors such as cyanide (1--10 mMol. in the surrounding artificial sea water), 2.4 dinitro-phenol (0.1--0.2 mMol.) or azide (3 mMol.). The Na 2~ efflux and K a2 influx increased once more when the inhibitor was removed. Experiments on the effects of these inhibitors on the amounts of ATP and arginine phosphate in squid axons showed that the arginine phosphate and then the ATP were broken down until they had practically disappeared from the axoplasm (CALDWELL 1958a(CALDWELL , 1960)). When the inhibitors were removed, ATP and in some cases arginine phosphate reappeared in the axoplasm. This indicated a connection between the ability of the axons to carry out active transport of sodium from the * Referat auf der 26. Tagung der I)eutschen Physiologischen Gesellschaft, Freiburg, Juni 1960.