[Illustration: FIG. 141.—A, Alimentary canal of Syllid: B, transverse section of pharynx of the same; _b_, buccal region; _d_, oesophageal outgrowth; _g_, salivary glands; _i_, intestine; _j_, tooth; _p_, pharynx; _s_, gizzard: C, alimentary canal of _Petta_ (after Wirén); _i_, intestine; _o_, oesophagus; _r_, rectum; _s_, stomach.] [Illustration: FIG. 142.—Alimentary canal of _Aphrodite_. × 1. (From Gegenbaur.) _a_, Anus; _b_, pharynx; _c_, caeca; _o_, mouth.] The intestine is looped in a few genera only, as in _Trophonia_, or coiled, as in _Sternaspis_, _Petta_ (Amphictenid, Fig. 141, C), and _Ammotrypane_. In the course of the tube there may be a thick-walled muscular _gizzard_, with hard chitinous lining, as in certain Terebellids, where it appears to replace, in function, the pharynx of the Nereidiformia; in the Syllidae the gizzard is present in addition to the pharynx (Fig. 141, A). Glandular appendages of the oesophagus are present in many {272}worms. Amongst the Nereidiformia, the Syllidae and Hesionidae possess oesophageal diverticula (Fig. 141, A, _d_), which are used, not for secreting a digestive fluid, but as reservoirs for water and air swallowed by the worms; and are provided with muscular walls, by which their contents can be driven out. They appear, in fact, to be used like the swim-bladder of fishes.[325] Many Chaetopods take in water by the anus—no doubt for respiratory purposes—and pass it forwards along the intestine. In the Capitelliformia a special groove conducts the water for some distance, then the groove becomes closed to form a canal, which, after a course forwards as a free tube below the intestine, again enters the latter, constituting a "siphonal apparatus," similar to that of the Echiuroids and the sea urchins. SENSE ORGANS.—In addition to the prostomial _eyes_, which are present in nearly all the Nereidiformia and Spioniformia, eyes may exist elsewhere on the body: thus _Myxicola infundibulum_ and _Fabricia_ possess a pair on the anal segment; in _M. aesthetica_ Clap. there is a pair to every segment; in _Branchiomma_ there is a compound eye near the tip of each gill filament (_i.e._ palp); whilst in _Dasychone_ a series occurs along each gill filament. All these examples belong to the Cryptocephala, in which, owing to certain peculiar modes of life, these sense organs are required in correspondingly peculiar positions. It is usually stated that _Polyophthalmus_ possesses, in addition to the usual prostomial eyes, twelve pairs on as many successive segments; but the minute structure of these organs points rather to their function as light-producing organs. [Illustration: FIG. 143.—A gill filament, A, of _Branchiomma_, B, of _Dasychone_. _a_, Axis; _f_, secondary filaments; _o_, compound eye; _x_, lappets.]