270 



POLYCHAETA 



CHAP. 



Amongst the Nereidiformia the jaws are absent in the Phyllo- 

 docidae 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 pre- 

 sent, as in Glycera (F). In Polyno'e the four jaws are carried 



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 ; /, 

 tooth ; k, base into which muscles are inserted : C, Polynoid ; U, upper, and L, 

 lower jaws ; /, tooth ; k, base : D, Diagrammatic section across pharynx of Auto- 

 lytus ; E, of Nereis ; F, of Glycera ; G, of Polyno'e. 



by hard pieces, to which the muscles are attached (C and G). In 

 Mphthys there is a dorsal and a ventral jaw. 



In the Eunicidae, however, the numerous denticles are carried 

 in a special pouch below the food tract, with which it communi- 

 cates anteriorly. 1 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 " (/) 

 at its anterior end. The upper series (U) consists of several 

 pieces, varying in shape and size in the various genera of this 



1 Compare with this the muscular organ of Dinophilus, p. 243, Protodrilus, and 

 a similar structure which occurs in Terebellids. 



