Intracellular studies on the cardiac ganglion cells of invertebrates have been confined mainly to the hearts of decapod and stomatopod crustaceans. The cardiac ganglia of these animals contain relatively few neurones, thought to be divisible into large follower cells and small pacemaker cells (for reviews see Hagiwara, 1961; Bullock & Horridge, 1965). Studies on follower cells of crustacean car...