[Illustration: 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.] {264}In a few cases, such as the Chlorhaemids and _Sternaspis_, and to a slight degree in _Arenicola_, the "head" and even the anterior part of the worm is capable of being withdrawn into the body. THE PARAPODIA AND CHAETAE.—The typical parts of a parapodium have been described in the preceding chapter; here it is only necessary to refer to the series of diagrams (Figs. 136, 137) representing the parapodia of the more common Polychaetes, and to add a few remarks about them. [Illustration: FIG. 136.—Parapodia. A, _Nephthys_; B, _Amphinome_; C, _Glycera_ (the unlettered lobe above _g_ is the notopodial cirrus); D, _Syllis_; E, _Eunice_; F, _Phyllodoce_. _a_, Notopodial cirrus; _b_, notopodium; _c_, neuropodium; _d_, neuropodial cirrus; _g_, special gill; _n_, aciculum (omitted in B); _x_, cirriform lip of chaetigerous sac.] In most Annelids the chaetae are in two bundles on each side, but there are certain families in which the dorsal bundle, and even the notopodium itself, is absent, as in the Eunicidae, Syllidae, and Phyllodocidae; or the dorsal bundle may be absent only in certain regions of the body, as in the hind-body of Terebellids. In some Amphinomidae and Aphroditidae the notopodium is scarcely distinct as a separate lobe, being a slight tubercle on the upper surface of the neuropodium; but the notopodial chaetae are present, and indeed particularly well developed in many cases. But whilst, in the Nereidiformia, the parapodia, whether {265}consisting of two lobes or only one, are always well developed, and project to a more or less pronounced degree from the sides of the body, it is otherwise in the rest of the group, where the chaetigerous lobes are usually reduced to mere tubercles or ridges, no doubt in relation to their burrowing or tubicolous habits. In _Sternaspis_ the chaetae issue directly from the body-wall. Amongst the Nereidiformia we find examples in which the parapodia, instead of being more or less conical "legs," are flattened fore and aft so as to serve as efficient "fins," as in the active swimmers, _Nereis virens_ and _Nephthys caeca_, and in the pelagic Phyllodocids, Alciopids, Typhloscolecids, and _Tomopteris_. [Illustration: FIG. 137.—Parapodia. A, _Polynoë_; B, _Scoloplos_; C, _Euphrosyne_. (Transverse section of body.) _a'_, Accessory cirrus; _y_, doubtful branchiae; D, _Sabella_ (thoracic). _a_, Notopodial cirrus ("elytron" in A, "gill" in B); _b_, notopodium; _c_, neuropodium; _d_, neuropodial cirrus; _n_, aciculum (accidentally omitted in C).]