The ALIMENTARY SYSTEM presents certain modifications of a systematic value. In the Nereidiformia the muscular pharynx, which is always protrusible and is preceded by an eversible buccal region, frequently encloses thickened cuticular plates which serve as crushing and grasping organs. The form, number, and arrangement of these "jaws" vary in the different families. They form valuable fossil records of extinct worms. In the Scoleciformia and Capitelliformia the buccal region exists, but there are no jaws. In the Sabelliformia and Terebelliformia eversion does not take place and jaws are absent. {270}Amongst the Nereidiformia the jaws are absent in the Phyllodocidae and Hesionidae; when present they are usually set in the direct course of the food. There may be one small tooth used for stabbing, as in some Syllids (Fig. 141, A); or a circle of such denticles (_Autolytus_, Fig. 140, D). To these are added powerful grasping jaws in _Nereis_ (E); or the latter may alone be present, as in _Glycera_ (F). In _Polynoë_ the four jaws are carried by hard pieces, to which the muscles are attached (C and G). In _Nephthys_ there is a dorsal and a ventral jaw. [Illustration: FIG. 140.—Jaws of various Chaetopods. A, Transverse section of the anterior end of _Eunice_; _a_, _b_, _c_, _d_, various parts of the upper series of denticles lying in a special chamber; _g_, oesophagus; _k_, lower jaw: B, the denticles of _Eunice_ separated; _U_, upper series; _a_, grinder; _b_, forceps; _c_, rasping plates; _d_, grater; _L_, lower series; _j_, tooth; _k_, base into which muscles are inserted: C, Polynoid; _U_, upper, and _L_, lower jaws; _j_, tooth; _k_, base: D, Diagrammatic section across pharynx of Autolytus; E, of _Nereis_; F, of _Glycera_; G, of _Polynoë_.] In the Eunicidae, however, the numerous denticles are carried in a special pouch below the food tract, with which it communicates anteriorly.[323] They are arranged in an upper and lower series. The lower series (L) consists of a pair of flat plates (_k_) on each side partially embedded in and acted upon by muscles, with a harder enamelled piece—the actual lower "tooth" (_j_)—at its anterior end. The upper series (U) consists of several pieces, varying in shape and size in the various genera of this {271}family; but developmentally they result from modifications of two rows of small, similar pieces.[324] The _intestine_ is generally straight and cylindrical, and is usually constricted by the septa, if these are present. In the Polynoids the intervening sacculations become so long as to receive the name of "caeca," which, in _Aphrodite_, become enormously elongated (Fig. 142); there are eighteen pairs of them (_c_), each being a slender tube bent upon itself, giving off short branches and dilated distally, where it lies in the base of the parapodium.