In the other sub-Orders the prostomium carries neither palps nor tentacles. {262}[Illustration: FIG. 134.—Heads of various Polychaeta (diagrammatic). A, Polynoid; B, Syllid; C, _Nephthys_; D, _Eunice_; E, _Phyllodoce_; F, _Trophonia_: _a_, prostomium; _c_, normal cirrus; _c^1_, peristomial cirri; _c^2_, cirrus of second segment; _c^3_, cirrus of third segment; _el^1_ 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 {263}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.