CHAP. V ENTERIC CANAL 327 



The digestive canal is not a simple tube of even calibre 

 throughout, but is divisible into several portions. The 

 mouth is bounded by a soft lip and leads into a small 

 buccal cavity, which communicates with a thick-walled 

 pha7j7tx (Fig. 79, pJi), extending through about five seg- 

 ments and connected with the body-wall by a number of 

 radially arranged muscle-fibres, the septa being absent in 

 this region. When the worm feeds, the buccal cavity is 

 everted, and the muscles serve to draw it and the pharynx 

 back again, as well as to dilate the pharynx. The latter is 

 followed by a narrow gullet or cesophagus {oes.) extending 

 through about eight segments, which is provided towards its 

 hinder end with three lateral pairs of glandular swellings of 

 a yellowish colour — the esophageal glands : these contain a 

 calcareous substance which serves to neutralise the organic 

 acids present in the food swallowed. The first two pairs 

 communicate with the third pair, which open into the 

 oesophagus {pes. gl). The gullet opens into a dilated, 

 thin-walled receptacle, the crop {cr), and this, again, com- 

 municates posteriorly with a large gizzard (giz) with thick 

 and muscular walls, which in about the 20th segment com- 

 municates with the intestine (int). The intestine has a 

 similar character throughout, and extends from the gizzard 

 to the anus : its dorsal wall is folded inwards so as to pro- 

 duce a longitudinal ridge or typhlosole (Fig. 78, typK), which 

 serves to increase the absorptive surface and in the interior 

 of which the yellow cells are very numerous. 



Certain of the cells lining the enteric canal, and especially 

 those along the typhlosole, are very granular, and like the 

 endoderm cells of the hypostome of Hydra (p. 298) are 

 to be considered as unicellular glands. They secrete a 

 digestive juice which — mixing with the various substances 

 containing organic acids taken in by the mouth, and 



