Segmental musculature and parapodial movement of Nereis diversicolor and Nephthys hombergi (Annelida: Polychaeta)

Abstract

The segmental musculature of Nereis diversicolor O. F. Müller is described. The musculature of Nereis irrorata (Malmgren), Nereis fucata (Savigny) and Neanthes virens (Sars) is essentially similar but comparison with Perinereis cultrifera (Grube) shows that, at least as far as the oblique muscles are concerned, the segmental musculature of all members of the Nereidae is not identical. A comparison is made with the musculature of Nephthys hombergi (Audouin & Milne Edwards). Swimming in Nereis and Nephthys is achieved by waves of lateral undulation passing from tail to head with associated parapodial deflection. The promotor and remotor muscles of the parapodium are the ventral parapodial obliques inserting at the parapodial base on anterior and posterior faces. This point of insertion is the hinge line of the parapodium. The dorsal parapodial muscles of Nephthys are more complex than in Nereis, apparently associated with the divergent parapodial rami of the former. In slow creeping, intrinsic muscles of the parapodium of Nereis raise it from the substratum and direct it forward, drawing in the neuropodium. Acicular muscles are responsible for its backward extension. The extended notopodium is a firm base for further protraction of the neuropodium. These muscles in Nephthys are modified for burrowing; the parapodia help to stabilize the body during proboscis eversion. Acicular muscles extend the parapodial rami dorsolaterally and ventrolaterally to grip the sides of the burrow. Intrinsic muscles withdraw the notopodial and neuropodial tips. The septum of N. diversicolor is not reduced in the typical body segment and its muscle fibres penetrate the dorsal longitudinal muscle to insert into the circular muscle layer. The pseudoseptum of Nephthys is probably homologous with that of Glycera and Phyllodoce and correlated with possession of a long, eversible proboscis. The complexity of oblique muscles in Nereis throws doubt on their alleged polyneuronal innervation. Muscular division of labour in slow and fast parapodial movements is proposed as an alternative to innervation by fast and slow fibres.

Confocal analysis of nervous system architecture in direct-developing juveniles of Neanthes arenaceodentata (Annelida, Nereididae)

“…He therefore de-emphasized the role of peripheral motor neurons in polychaetes, and concluded that one fast, one slow, and one inhibitor motor neuron triply innervate this muscle, similar to certain arthropod muscles. Mettam [45], however, downplayed the importance of polyneuronal innervation. He pointed out that Dorsett's [51] parapodial retractor is actually three muscles: the posterior parapodial obliques.…”

“…He pointed out that Dorsett's [51] parapodial retractor is actually three muscles: the posterior parapodial obliques. These function in rapid parapodial deflection during swimming and thus do not require slow motor innervation [45]. Efforts to establish a clearer understanding of polychaete parapodial ganglia and motor-neuronal arrangements for somatic musculature will benefit from the fine-scale resolution and efficiency of CLSM.…”

Expression of the Lhx genes apterous and lim1 in an errant polychaete: implications for bilaterian appendage evolution, neural development, and muscle diversification

“…Parapodial cirri are sensory, whereas ligules are chiefly respiratory but apparently also capable of sensation [48,51]. Chaetae emerge from between the chaetal lobes and are anchored basally within an internal chaetal sac ensheathed by muscles that move the sac along an internal support rod (the acicula) to effect chaetal protraction and retraction [52]. …”

Biomimetics and the case of the remarkable ragworms

“…During slow crawling, no (or only very insignificant) lateral undulations of the body are observed and each parapodium functions similarly to a leg in terrestrial animals. It is lifted from the ground and drawn inward and forward in the recovery stroke where after the tip of the parapodium touches the ground and forms a fulcrum (although some slippage is normally observed), around which the parapodium turns during the power stroke and thus advances the segment (Mettam 1967;Gray 1939;Foxon 1936). During fast crawling, the above described parapodial movement is coupled with lateral undulations of the body travelling in a posteroanterior direction.…”

“…The tip of the parapodium touches the ground as the segment on which it is attached approaches the crest of the body wave, so that the backward power stroke of the parapodium coincides with the crest of the body wave. This means that the force, generated during the power stroke, does not only stem from the parapodial muscles but is amplified by the longitudinal muscles and their resulting body movements (Mettam 1967;Gray 1939). During swimming, the body is no longer in contact with the substrate and the number of body waves along the body decreases and the waves have larger amplitudes (Clark and Tritton 1970).…”