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.

With 10 figures in the text)The body form of Aphrodiie is attributed to its exploitation of the nereid creeping mechanism. It employs a fast stepping pattern, individual limb movements being extension and retraction of the neuropodiuni with co-ordinated chaetal movement. The downward and backward propulsive stroke raises the worm clear of a hard substratum. The notopodium is stabilized and its long chaetae raised and lowered in defence.The segments are short but wide. The body wall muscles, relieved of locomotory function, are a basketwork of longitudinal and diagonal muscles capable of maintaining constant body dimensions. Muscles of the appendages originate from stabilized points on the body wall. Trunk septa are reduced, in association with the constant body shape, but the first few septa form a membrane enclosing the proboscis. Complete and muscular septa occur in the rectal segments.During respiratory movements the elytra are depressed by intrinsic and extrinsic elytrophore muscles and elevated by coelomic pressure. Coelomic pressure drops immediately before elytral depression and is controlled by the body wall muscles, especially the diagonal muscles.Aphrodite differs from its near relative Hermione in the reduction of septa and simplification of body musculature.

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Until recently, the most detailed and widely quoted account of the life cycle and reproductive biology of Nereis diversicolor O. F. Müller was that of Dales (1950; 1951) for a population in the Thames Estuary followed for a single season. The study combined a periodic analysis of the population structure with assessment of reproductive maturity, by gamete analysis of individual worms, and a record of larval recruitment. The conclusions reached were that worms grew to maturity in one year, spawning in February, but that some survived up to a maximum life span of 18 months.

Nereid polychaetes are among the most abundant worms in the estuaries of north-western Europe. Like most other estuarine macrofauna, they have their origins in marine conditions and the colonisation of estuaries has presented special challenges, both physiological and ecological. This paper considers some of the special strategies which appear to have accompanied the penetration and colonisation of estuaries by nereids.