THE PHYLUM CHORDATA 



37 



nute organic particles, such as protozoa and pelagic larvse. The 

 mechanism for concentrating this dilute food is somewhat complex. 

 The pharynx is lined with ciHated epitheUum and the ciUa beat 

 inward so as to 

 cause a current of 

 water to be drawn 

 into the mouth and 

 out through the 

 gill-sHts. The ciUa 

 beat also down- 

 ward, so as to force 

 the cvurent to the 

 floor of the phar- 

 ynx, where there is 

 a hypopharyngeal 

 groove or endostyle 

 (Fig. 10) fiUed with 

 sticky mucus to 

 which food parti- 

 cles adhere. The 

 endostyle is pro- 

 vided with strong 

 cilia which whip 

 the mucus into a 

 rope-hke mass and 

 drive it forward 

 with its burden of 

 food particles to 

 the anterior end 

 of the phamyx. 

 There the endo- 

 style bifurcates 

 aroimd the mouth 

 in the form of 

 two semicircular 

 grooves, the peri- 

 ■pharyngecd bands, which unite again above the mouth and form 

 the dorsal or hyperpharngeal groove (Fig. 10) . The mucous rope travels 

 backward in the dorsal groove till it reaches the stomach and intes- 



en 



Fig. 10. — ^Transverse section through the pharyngeal 

 region of Amphioxus. a, atrial cavity; c, ccelomic cav- 

 ity; df, dorsal fin fold; en, endostyle; /r, fin ray; I, liver 

 diverticulum; to, myotome; mc, myocomma; mf, meta- 

 pleural fold; n, notochord; nc, nerve cord or spinal 

 chord; ne, nephridium; p, pharynx; pc, pharyngeal cleft; 

 sg, supra-pharyngeal or dorsal groove. (Redrawn and 

 modified after Lankester and Boveri.) 



