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 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.
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.
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 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.