The first ventral or sub-oesophageal ganglion is contained in the peristomial somite. It may give off one, or two (Eunice), or three (Phyllodoce) pairs of nerves indicating a fusion of somites. Actual fusion of a setigerous somite in the young animal with the true peristome has been observed, e.g. in Nereids; cf. Langerhans, Z. W. Z. xxxii. 1879, p. 517. The appendages of the peristome may be parapodia with setae, or cirri usually much lengthened, and then termed by Claparede tentacular cirri.
1 A few setae are found at the base of the dorsal cirrus in Eunicidae. Tufts of fine setae podium and a ventral neuropodium, which are separated by a greater or less interval and differ in size and form not only in different Polychaeta but in different regions of the same individual. They may be absent in certain regions in some Tubicola, e. g. from the posterior part of the body in Arenicola, etc. The cirri are in intimate relation with the parapodia, and there is usually a dorsal or notopodial cirrus and a ventral or neuro-podial. The latter is as a rule insignificant in size. The cirri of the peristome, and following somites, and of the pygidial somite, are frequently much lengthened and enlarged. The organs in question are solid processes of the body-wall, probably tactile in function, with muscular walls, and containing a nerve. They vary in form, and may be conical, filamentous, lamellate, and are sometimes jointed. The Aphroditidae are distinguished among Polychaeta by the presence of lamellate plates or elytra attached dorsally to the notopodium.
They contain a rich supply of nerves and have been regarded as modified cirri, but the two structures sometimes co-exist on the same somite1. The branchiae, like the cirri, are processes of the body-wall, but they differ from them either by containing an extension of the coelome, or a single contractile vessel, or an afferent and an efferent vessel which are connected by capillary loops or networks. They are either filamentous, lamellate, pectinate, or branched in shape, and are present either on the majority of the somites, or on certain somites only in Tubicola, e. g. the anterior in Terebella or the median in Arenicola. They are often entirely absent. The prostomial tentacles of Terebellidae must possess a respiratory function, inasmuch as they contain a cavity which is an extension of the coelome. The cephalic gills of the Serpulidae consist of a more or less distinct basal ridge, the two ridges right and left being connected dorsally, from which spring a number of filaments, varying from 4-50. These filaments usually support each two lateral rows of ciliated processes. They may contain only an extension of the coelome, with the exception of the two median dorsal filaments, which are traversed by a single vessel, e. g.
Haplobranchus, Manayunkia; or all alike are traversed by a similar single vessel which gives off a branch into each lateral process. These vessels contract and dilate alternately. In the tribe Sabellinae both ridges and filaments are supported by a cellular cartilage and the filaments are connected by a membrane. In the Serptdinae, with few exceptions, e. g. Protida, one, rarely both, of the median dorsal filaments is modified into a stopper-like operculum, which closes the tube when the animal appear in the same position in the sexually mature Syllidae and Hesionidae, Pruvot concludes therefore that the simple or uniramose character of the pseudopodia in these worms is due to the atrophy of the notopodium.
It is a remarkable fact that the only setae found in Tomopteris are the single setae lodged in the extremities of the cephalic appendages. See Greeff, Z. W. Z. xxxii. 1879, Taf. xv. Figs. 40, 41, 47, The large pair of appendages are innervated from the cerebral ganglia (Pruvot).
1 For the structure of the elytra in Polynoe, see Jourdan, Z. A. viii. 1885.
The body-walls consist of a chitinoid cuticle, a hypodermis and muscular layers, a circular and a longitudinal. The cuticle is thin in Tubicola. The hypodermis consists of a single layer of cells, the outlines of which are sometimes lost (?). The cells are partly supporting cells, partly sensory, partly glandular. Gland-cells, the secretion of which contains rod-like bodies, as in Nemertea, are very common, especially in tubicolous families, e. g. Chaetopteridae, but are by no means restricted to them. The rod-like bodies have been found in the walls, of the tube of Sabella. Protective tubes are manufactured by some Errcuitia and all Tubicola. In the Errantia and some Tubicola, e. g. Pectinaria, the tube is carried about by the animal; in other Tubicola it is formed in sand or mud or is attached to stones. It may be soft in consistency, e. g. Siphonostoma; tough and parchment-like, e. g. Chaetopterus; or stony, as in Serpulidae. It is often strengthened by particles of mud, sand, or shell, e. g. Sabella, Hermella, Terebella. The material for the tube is probably secreted by the glands of the body-surface2. Cilia are found on the prostomium of the Oligochaete Aeolosoma and in Polychaeta chiefly on the sides of the prostomium or somites, round the anus, or on the branchiae.
Ophyotrocha puerilis (Eunicidae) retains the polytrochal rings of cilia (infra, p. 606). The Sabellinae possess a ventral ciliated furrow in the abdominal region, which either ends at the thorax, or passing round its right side, extends along its dorsal median line. The ventral surface, rarely the dorsal, of Serpidinae is ciliated but not furrowed. The cilia in both instances convey faecal particles forwards and out of the tube. The circular layer of muscles may be absent, e. g. in Nephthys, or be sparingly developed. The longitudinal layer is commonly disposed in Polychaeta in four bands, two latero-dorsal, two latero-ventral. The internal ends of the setiparous sacs penetrate and therefore divide the corresponding layer in Oligochaeta. The setiparous sacs have special protrusor muscles, derived in Oligochaeta from the circular layer. Muscular bands pass from the median ventral line in Polychaeta, in an oblique dorso-ventral direction and are inserted into the dorsum, the parapodia, and the setiparous sacs. They consequently divide the coelome into three longitudinal chambers, a median containing the digestive tract, and two latero-inferior1. It is also divided by transverse fibro-muscular septa into a series of more or less complete chambers, one behind the other.