[Illustration: 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 {261}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_). [Illustration: 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.] In the family Serpulidae one (rarely two) of the most dorsally placed gill filaments is enlarged terminally, and acts as a stopper or "operculum," which closes the mouth of the tube when the animal withdraws into it. Further, in _Spirorbis_ this operculum is grooved on one side, and serves as a brood pouch in which the eggs undergo development (Fig. 184, p. 341). It will be seen, therefore, that the palps may be very important organs for the life of the worm, and they are no less interesting to the comparative anatomist, serving as they do as an excellent illustration of the various uses which Nature finds for one and the same organ.