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.
In the other sub-Orders the prostomium carries neither palps nor tentacles.
Fig. 134.—Heads of various Polychaeta (diagrammatic). A, Polynoid; B, Syllid; C, Nephthys; D, Eunice; E, Phyllodoce; F, Trophonia: a, prostomium; c, normal cirrus; c1, peristomial cirri; c2, cirrus of second segment; c3, cirrus of third segment; el1 point of attachment of elytron; p, palp: s, nuchal organ (ciliated pit); t, tentacle; I, peristomium; II, III, IV, segments.
The tentacles in the Nereidiformia present a wide variation in number; probably the typical number is three, one of which is median and two lateral—as in Polynoids, Syllidae, and some Eunicidae. Further, there is a certain amount of evidence in the nerve supply of the median tentacle to show that it was originally double. The presence of four tentacles, then, as in Nephthys, Phyllodoce, and Glycera, may be a primitive condition. By the disappearance of the paired lateral tentacles the worm possesses a single median one, as in Aphrodite and Amphinomids;[[317]] whilst a duplication of these lateral ones leads to the condition of Eunice and Hyalinoecia, which have five tentacles. In the Chlorhaemidae the number is further increased to five or more on each side,[[318]] and in the Terebellidae these prostomial processes become very numerous.
In the Cryptocephala there is never more than a single pair of tentacles, and these are generally reduced to a group of sensory cells, though in Sabellaria they retain a considerable size.
In a few genera, such as Aphrodite, Nephthys, Capitella, the first postoral segment is distinguished from the succeeding segments only by its position with regard to the mouth (Fig. 132) and by its smaller size. But in the remainder of the Polychaeta, with here and there an exception, the peristomium is achaetous in the adult.[[319]]
Except in the Nereidiformia, peristomial or tentacular cirri are rare, being represented in the Spioniformia by the very long "tentacles." In the Nereidiformia one or more of the following segments may be added to the peristomium, and share in the "cephalisation," which is so characteristic a feature in this group. In Amphinomids the first three or four chaetigerous segments are incomplete ventrally, owing to the shifting of the mouth backwards; these segments form lateral lips, but they are not otherwise modified. In Phyllodoce, however, there are four cirri on each side of the mouth, and from the arrangement in the Alciopids we are justified in concluding that the segment which carries the four pairs of cirri is really made up of three segments (Fig. 134, E). Among the Hesionids there are four such "cephalised" achaetous segments with long cirri.
Fig. 135.—Sabellaria alveolata L. Ventral view of anterior region, × 10. a, Notopodial cirrus; b, notopodium; c, neuropodium; ch, peristomial chaetae; d, neuropodial cirrus; m, mouth; P, multifid palp (gill filaments); P', ridges after removal of gill filaments; s, ventral (tubiparous) gland shield; T, tentacle; I, hood formed by peristomium; II to VI, following segments.