The posterior extremity is generally more or less narrowed, and most of the Nereidiformia are provided with special elongated cirri, borne by the anal segment. In the Maldanidae and others the body terminates in a funnel, at the bottom of which is placed the anus. Only in a few cases is the anus not terminal; in Notopygos and other Amphinomidae, as well as in some species of Polynoë, it is dorsal.[[313]] In Sabellaria and Pectinaria the hinder end of the body undergoes great degeneration; in the former it is achaetous, but cylindrical and bent forwards alongside the body (Fig. 131). In Pectinaria (Fig. 177), this region, which is called the "scapha," is leaf-like, and serves to close the narrower end of the tube in which the worm lives. Arenicola marina, and some Terebellids have no chaetae in the hinder, narrower part of the body.
Fig. 131.—Sabellaria alveolata L. × 3. (After Malmgren.) a, Anus.
The Head.—The prostomium is, in the majority of cases, rounded or conical, though it may be square (Nephthys) or elongated and jointed (Glycera), or even hammer-shaped (Tomopteris); or it may be fused with the peristomium, and apparently absent (Arenicola). In the great group Cryptocephala, the peristomium grows forwards so as to hide the prostomium entirely.
In a few of the Nereidiformia the prostomium is compressed, and in the Amphinomidae it is provided with a dorsal ridge or "caruncle," which is a leaf-like process overlapping three or more segments. In many Aphroditidae (as well as in Polydora) there is a peculiar "frontal" ridge passing forwards from the prostomial tentacle, and downwards into the mouth (Figs. 132, c, and 133, A, x).
Fig. 132.—Aphrodite aculeata L. Ventral view of anterior region, × 6. a, Prostomium; c, frontal ridge on prostomium; d, neuropodial cirrus; l, lower lip; m, mouth; p, palp; s, intersegmental groove; t, tentacle; I, foot of peristomium, which has shifted forwards so as to lie in front of the mouth; II to V, successive feet.
In all the Nereidiformia, as well as in Sabelliformia and Chlorhaemidae, the prostomium bears sensory processes of two kinds, viz. dorsal tentacles and ventral palps. The latter are invariably two in number, and are particularly well developed in Aphroditidae, Nereidae, Syllidae, some of the Eunicidae, and in Chlorhaemidae. Even when they are apparently absent, as in Nephthys, it is possible that they are represented by certain lobes at the sides of the mouth, for in many Syllidae they are so fused with the prostomium as to be scarcely distinguishable. In the Chlorhaemids the palps[[314]] are grooved, and in the Sabelliformia they become considerably branched, and extend round the prostomium so as to nearly meet dorsally and ventrally. Each palp is, in this sub-Order, represented by a greater or smaller number of long, mobile filaments, arising from a common base; they are grooved along the inner side, ciliated, and provided with secondary processes. The crown of "gills," in fact, is nothing more than the greatly subdivided and enormously elongated palps, as both Pruvot[[315]] and Meyer[[316]] have shown. In such forms as Haplobranchus and Amphicorine the process of subdivision (branching) has only gone a short way. In all the Sabelliformia each filament, in addition to its sensory function, aids in conveying food to the mouth by the action of the cilia, and has a blood-vessel within, thus acting as a respiratory organ. The filament may carry compound eyes (Fig. 143) either at its apex (Branchiomma) or at intervals along its course (Dasychone).
Fig. 133.—A, Anterior end of Polydora enlarged. a, Prostomium; x, frontal ridge; I, peristomium; c', its long cirrus; II, III, etc., the following segments; c, gill; B, head of Sabellid; P, palps (branchial crown); t, position of tentacles; l, processes of upper lip membrane; I, peristomium raised into a collar; II, III, IV, following segments.