Giant Nerve Fibres in Maldanid Polychætes

Abstract

GIANT nerve fibres in polychætous annelids have been defined as those which are “disproportionately greater in size than the other nerve fibres in the animal”, and which are “concerned with quick escape or withdrawal movements, effected by widespread and synchronous muscular contractions”¹. The definition is based on anatomical evidence, which has been accumulating for more than a century, of the widespread occurrence of giant fibre systems in many polychæte families, and on the behavioural correlation of rapid withdrawal responses in species possessing giant fibre systems. Notable among the physiological evidence is the work of Bullock², who recorded giant spikes from ten species representing ten families, and concluded that the motor response following giant discharge is generally a twitch-like withdrawal. In the context of our findings reported here, it is interesting that he obtained preliminary evidence for a rapid pathway in Arabella iricolor, although this worm possesses neither a long distance giant fibre system nor a rapid withdrawal response.