They may be absent also in some Polychaeta and rudimentary in others, and lodged either within, or in close apposition with, the cerebral ganglia. As a rule they are confined to the prostomium, but among Tubicola Polyophthahnus possesses in addition to cephalic eyes a number of lateral eyes (twelve pairs in P. pictus), one pair to a somite, situated ventrally to the insertion of the oblique dorso-ventral muscles (supra): Leptochone aesthetica (Eriographidae among Serpulidae) has a pair on each somite; Fabricia (Sabellinae) a pair on the anal somite; and many species of Sabclla (sometimes distinguished under the generic name Branchiommd) and the genus Dasychone have them on the branchial filaments.
1 It forms in Eunicidae a sub-pharyngeal ganglionated cord and an oesophageal ring. It has in Nephthydae and Phyllodocidae long roots, which end in a ring of minute ganglia. Full details are given by Pruvot.
2 Ganglion cells are found in connection with the grooves in Polyophthalmus. A pair of grooves is found on each segment of the body in Aricia.
Each eye of Nereis cidtrifera consists of a hollow sphere filled with a coagulable fluid. Its walls are composed of a single layer of cells: the cells of the anterior or outer aspect are flattened and clear, those of the posterior or inner aspect or retinal cells, to which the nerve is distributed, elongate, and of two kinds commingled, one pigmented, the other not pigmented. The hypodermis cells immediately above the eye are flattened; around it, of great length, whilst the cuticle is unchanged. There is a special optic ganglion for the anterior of the two pairs. The structure of the two eyes of Alciope is similar but differs in the following points. The inner ends of the retinal cells are capped by a rod (or two semi-cylinders) and pigment (? in special cells) is massed at the spot where the rods commence; a spherical lens lies to the outer side of the cavity of the sphere from which it is separated by a membrane. The cephalic eyes of Polyophthahnus, which are imbedded in the cerebral ganglia, consist of three clear bodies contained in a pigmented cup; its lateral eyes possess a hyaline lens, a layer of prismatic cells and nervous fibres inclosed in a pigmented capsule, and both cuticle and hypodermis are thinner where they pass over them.
The eye of Hyalinoecia tubicola is said to consist of a lens-like thickening of the cuticle with two layers of cells applied to it (Pruvot). Otocysts are comparatively rare. A pair is found in the pro-stomium of some Tubicola, e.g. Arenicola, Fabricia, some Sabellinaes and young Terebellidae; two or three pairs in the first setigerous somite of Leptochone. Three lie near the eyes in the cerebral ganglia of Polyophthahnus; and the genus Aricia has a pair dorsally placed in a variable number of the body-somites. The otocyst appears to consist of a capsule composed of ciliated cells and containing one or more calcareous otoliths suspended in a fluid.
The mouth is sometimes terminal, more often ventral. It is a transverse or longitudinal slit guarded by two lateral, or by upper and lower* lips, and leads into a stomodaeum, which forms a buccal cavity, and in most Polychaeta Errantia, some Tubicola, as well as in Oligochaeta, a muscular pharynx. The latter is frequently more or less protrusible. Its-epithelium is sometimes ciliated and is generally covered by a cuticle: its inner surface is sometimes papillate (e. g. in Syllidae), and may be armed with chitinoid teeth, either flattened toothed plates (Eunicidae) or pointed cones (Aphroditidae, Nereidae). Some Syllidae possess a pharyngeal tooth traversed by the duct of a paired poison gland (Langerhans). Glands may be imbedded in the walls of the pharynx or appended to it externally. The archenteric region is divisible into an oesophagus and a stomach-intestine. The length and structure of the oesophagus vary much. In the terrestrial Oligochaeta it is sometimes provided with a muscular gizzard, a crop, and lime-secreting glands of Morren (p. 202). The glandular stomach so-called of Syllidae immediately following the oesophagus is really a muscular gizzard with radial columns of striated muscles in its walls (Haswell). In some Syllidians and in Hesionidae it is furnished with a pair of outgrowths which contain air, probably secreted from the blood and derived in the first instance by the blood from swallowed air1. The stomach-intestine is larger in calibre than the oesophagus, and is usually more or less constricted by the attachment of the septa or of muscular bands representing them.
It is also usually pigmented. The pigment is contained in the enlarged peritoneal cells ('chloragogen' cells) which form its outer coat, and very rarely in the lining epithelium. A pair of glands may open into its commencement (e. g. Arenicold), or a variable number into its dorsal aspect (some terrestrial Oligochaeta, p. 202); and in Aphrodite each of its segments has appended a right and left glandular caecum branched at its distal extremity. The Capitellidae possess a 'siphon' or accessory intestine ventral to the main intestine and opening into it anteriorly and posteriorly. A median longitudinal invagination of all its coats forms the typhlosole of some terrestrial Oligochaeta (p. 203). The coats of the oesophagus and intestine are an outer peritoneal cellular coat; an outer longitudinal and inner circular muscle-layer, sometimes, but very rarely, reversed in position; and the epithelium. The latter is usually ciliated and often contains glandular cells. In the intestine its cells may grow in size, be thrown off, and replaced by fresh cells as in some Oligochaeta (p. 203)2. There is a short rectum, probably always derived from a proctodaeum. The anus is usually terminal, sometimes dorsal.
The alimentary canal is as a rule straight, and rarely disposed in coils, e. g. in Pectinaria and Siphonostomum among Tubicola. It is nearly always supported by a dorsal mesentery which incloses the dorsal bloodvessel. The ventral mesentery sometimes disappears completely, sometimes persists so far as to support the supra-nervian vessel.