250 



POLYCHAETA 



out the following chapters the word " buccal " region is used for 

 that part if any which is thus everted (Figs. 125, 126). 



Both the buccal and pharyn- 

 geal regions are wrapped round 

 by several coats of muscle, to 

 form apparently a single mus- 

 cular organ (Fig. 127, sJi), which 

 occupies about eight segments 

 in a condition of complete in- 

 troversion. The septa are absent 

 from the anterior part of the 

 body. 



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 arrange- 

 ment 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. 



E version 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 



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, rt - 

 tractors. 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. 



