136 ORGANS AND ORGAN SYSTEMS 



stomach-intestine opens to the outside through the anus by a 

 short section of non-functional tissue called the proctodaeum. 



In the cavities between the digestive tract and the body wall 

 lie all of the other important organs of the worm. They are, 

 therefore, morphologically speaking, inside the worm while 

 undigested food, dirt, etc., although inside the digestive tract, 

 are morphologically, outside of the animal. Some of these 

 internal organs, like those of the excretory system are repeated 

 in each somite, others,, like the blood, vascular and nervous 

 systems, are continuous from one end of the body to the other, 

 while still others, like the reproductive system, are concentrated 

 in one part, occupying only a few somites. 



D. PHYSIOLOGY OF THE DIGESTIVE SYSTEM. Buccal Cavity 

 and Pharynx. The mouth is covered by a dorsal prolongation 

 of the first somite functioning as an upper lip or prostomium. 

 A much smaller under lip completes the border. The buccal 

 cavity is a relatively spacious hollow reaching as far as the third 

 somite and its walls are provided with special muscles reaching 

 to the body wall. This cavity opens into a larger and more 

 muscular pharynx. The lips do not act as grasping organs for 

 ingestion of leaves, but the prostomium is rather an organ of 

 smell, while the pharynx with its heavy muscles acts as a suc- 

 tion pump for drawing leaves into the burrows and for ingesting 

 them afterward. The leaves line the walls of the burrows or 

 partly extend out of them masking the openings. If such a 

 partially visible leaf is pulled out the part inside the burrow will 

 be found to be a mere skeleton, the mesophyll structures having 

 been sucked into the pharynx of the worm by the action of the 

 muscular pump. This action is facilitated by the secretion 

 from the mouth and pharynx of a digestive fluid of alkaline 

 reaction which, however, only softens and does not digest, 

 although Darwin suspected the presence of tryptic protease and 

 amylytic ferments. 



The Oesophagus. The oesophagus stretches from the sixth to 

 the thirteenth somites; it is somewhat laterally compressed with 

 longitudinal and annular muscles. In the posterior part of the 

 oesophagus the walls are considerably thickened to form three 



