584 ROBERT MATHESON 



tracheal system which is in direct connection with the rich in- 

 coming air supply. The oesophagus requires only a very small 

 amount of oxygen and consequently has a poorly developed tra- 

 cheal system. 



The pharynx and its muscles are supplied by a small tracheal 

 branch (fig. 1, a) which arises from the exterior longitudinal 

 trachea at its point of entrance into the head. Several other 

 small tracheae from the anterior branches of the main longitu- 

 dinal trunks supply the cephalic end of the pharynx and mouth- 

 parts. 



The pharynx 



That portion of the fore-gut, extending from the mouth to 

 near the posterior margin of the head, may be designated the 

 pharynx (fig. 1, ph). It is provided with special muscles and, 

 when expanded, is somewhat larger than the oesophagus. On 

 the latero-ventral surface on each side, just behind the sub- 

 oesophageal ganglion, are situated two series of small muscles. 

 These extend in a latero-caudad direction to their points of origin 

 on the posterior margin of the head (fig. 16). The number of 

 these muscles varies, the usual number on each side being from 

 16 to 19. They lie directly ventrad of the large tracheae which 

 penetrate the head. Directly in front of these lateral-inferior 

 muscles and on the ventral surface there is a series of three to 

 four small muscles on each side of the median line, which pass 

 almost directly ventrad to a short chitinous apodeme near the 

 labial region. 



On the dorsal side of the pharynx, directly above the lateral 

 series of muscles, are found two series of small muscles on each 

 side of the median line (fig. 15). The inner series includes 

 five, the outer four, muscles. They are attached on each side of 

 the middle line of the caudal portion of the vertex (fig. 15). 



The function of the pharyngeal muscles seems to be that of 

 supporting and enlarging the pharynx during feeding. On open- 

 ing a freshly etherized specimen it is easy to watch their action. 

 The pharynx in this region is seen gradually to expand, then a 

 wave-like contraction begins at the mouth and passes down the 



