The Nervous Anatomy of the Body Segments of Nereid Polychaetes

• Published in: Philosophical Transactions of the Royal Society of London, Series B: Biological Sciences Series B: Biological Sciences, 1957-01, Vol.240 (671), p.135-196
• Main Author: Smith, J.e.
• Format: Article
• Language: English
• Subjects: Axons; Epithelial cells; Ganglia; Innervation; Integument; Motor ability; Nerves; Neurons; Neuropil; Proprioception
• ISSN: 0962-8436; 0080-4622; 1471-2970; 2054-0280
• DOI: 10.1098/rstb.1957.0001

The nerve cord of nereid polychaetes consists of intersegmental ganglia linked by narrower connectives. Each ganglion gives rise to four pairs of peripheral nerves designated in their order of origin IV, I, II and III, but numbered I-IV in their segmental succession. Nerve I arises from the cord immediately behind the intersegmental septum, II (the parapodial nerve) and III leave the posterior end of the ganglion near the middle of the segment and IV originates from the anterior (preseptal) part of the succeeding ganglion at the posterior margin of the segment. Nerves I and IV cross the floor of the body wall transversely and terminate in the dorsal integument, II supplies the parapodium and III links ipsilaterally with homologous nerves of other segments through a lateral nerve which runs longitudinally in the ventral body wall adjacent to the bases of the parapodia. Nerves II are the largest, IV are next in size while I and III are very fine and visible only after staining. All the nerves are mixed and contain relatively few fibres. Each, on the afferent side, supplies a determinable region of the integument, I and IV between them drawing on integumentary receptors over the greater part of the ventral and the whole of the dorsal surface. Nerve II alone receives excitation from the parapodial integument and III is primarily proprioceptive, fibres entering the nerve from the surface of the dorsal and ventral longitudinal muscles. Sensory cells are most numerous in the parapodia, particularly in the cirri, and are present in large number in the ventral body wall. There are very few in the dorsal integument. Almost all are bipolar, usually single but occasionally grouped. Two morphological types of sensory cell are described. The internal (centrifugal) fibres of the sensory cells either run directly into the segmental nerves or, more frequently, discharge excitation into the nerve through tracts of a lattice-like subepithelial plexus made up of fibres of multipolar association cells. Excitation originating in scattered receptors thus appears to be canalized into the few fibres of the main nerves by way of the plexus. The internuncial systems of the cord through which the afferent (and efferent) fibres make their central connexion are of two kinds, (1) giant-fibres and (2) fine-fibres. The paired lateral and paramedial giant-fibres and the single median dorsal giant-fibre have a similar arrangement and distribution in Platynereis dumerilii and Nereis diversic