Squid escape by contracting the mantle to jet water. Firing every mantle muscle at once requires the signal to arrive everywhere simultaneously, and since conduction speed rises with axon diameter, squid evolved giant axons formed by fused cells, reaching around half a millimetre to a millimetre across.
That size made the fibre an instrument. In the late 1930s and 1940s, working largely at Plymouth on loliginid squid, Alan Hodgkin and Andrew Huxley threaded electrodes inside the axon and recorded the voltage across the membrane directly. Their measurements showed the action potential overshoots zero rather than merely collapsing the resting potential, and led to a quantitative model of sodium and potassium conductance that earned the 1963 Nobel Prize in Physiology or Medicine, shared with John Eccles.
Two clarifications are due. The giant axon is not unique to Loligo vulgaris; several squid species have been used, and the literature often says simply the squid giant axon. And its size reflects the demands of escape jetting, not any special intelligence, which is an inference sometimes drawn but not supported.