[Illustration: FIG. 126.—Diagrams to illustrate the action of the Chaetopodan "introvert." (From Lang.) A shows the apparatus at rest; the mouth leads into the buccal cavity (_B_), with paragnaths in its wall; _P_, the pharynx, with jaws at its anterior end; _c_, brain; _p_, protractor muscles; _r_, retractors. B, the pharynx has been brought forward or protruded by the eversion, or turning inside out, of the buccal region, so that the jaws (_j_) now lie some way in front of the head, which is represented by the brain.] The buccal region is lined with chitin, which is specially thickened at certain definite spots, forming small "denticles" or "paragnaths" (Fig. 125), which have a different arrangement in the various species. The cavity of the pharynx is narrow and the walls thick and muscular; each side wall carries a large, dark, chitinous "jaw" (Fig. 127, J), which is hollow at the base, into which the muscles serving to move it are inserted, whilst the apex is solid, curved, and more or less notched. These two great jaws are used not only for tearing prey, but for seizing it; for when the pharynx is entirely protruded the two jaws are wide apart, and when retraction takes place they come together and grasp the prey. Eversion of the apparatus is partly effected by protractor muscles (Fig. 126, A, _p_) and partly by the pressure of the coelomic fluid, compressed by the muscles of the body-wall; the eversion is stopped at a certain stage by a sheet of muscular tissue or "diaphragm" (Fig. 127, _diaph_) inserted round the buccal region and attached to the body-wall in the second segment. The introversion is effected partly by the contraction of this diaphragm and partly by the action of powerful retractor muscles (Fig. 126, _r_) inserted into the hinder end of the pharynx and passing to the body-wall (these are removed in Fig. 127). The {251}movement of the jaws themselves and of the wall of the apparatus is due to other muscles. The _oesophagus_ is quite short; into it opens a pair of sacculated diverticula or glands. Then follows the _intestine_, which extends through the rest of the body as a thin-walled tube, slightly dilated at the insertion of the septa.