The longfin inshore squid has the largest axons of all animals (around 1 mm in diameter). They were crucial in understanding neurons and the mechanism allowing the signal to travel down the nerve. By connecting squid's fins to an iPod with wires from earbuds the scientists were able to initiate action potential in the neurons involved in changing squid's camouflage.
https://blog.backyardbrains.com/2012/08/insane-in-the-chroma...
https://en.wikipedia.org/wiki/Action_potential#Experimental_...
https://en.wikipedia.org/wiki/Longfin_inshore_squid