n8 



The Pharynx. The pharynx, which is lined 

 throughout its extent with chitin, passes upwards and 

 backwards through the ganglionic ring formed by the 

 supra and infra-oesophageal ganglia and their com- 

 missures. At first it is narrow, but posteriorly becomes 

 a large chamber (the pumping organ) (Fig. 52). 



The pumping organ occupies with its muscles a 

 large portion of the head behind the level of the 

 cerebral ganglia. In the state of rest its lumen is 

 triradiate in transverse section. The walls are formed 

 of three large and thick chitinous plates, one placed 

 on either side, and one superiorly. Into each of these 

 plates powerful muscles are inserted. The plates are 

 connected by thin non-chitinous membrane, and their 

 edges are rolled so that they form a spring capable of 

 returning to their original position so soon as the 

 separating force of the muscles ceases. 



Posteriorly, where the pharynx becomes very 

 narrow, a sharp bend occurs and a valvular action is 

 produced. The whole forms a very powerful suctorial 

 apparatus. 



The Oesophagus. Immediately beyond the pump- 

 ing organ the chitinous layer ceases, and the rest of the 

 fore-gut is formed of excessively thin membrane. At 

 the junction of the two portions a sharp bend occurs, 

 and the floor projects so as to form a valvular flap. 



The thin-walled oesophagus is a large dilated sac, 

 whose walls are supported by surrounding structures. 

 Into the posterior wall of the dilated and thin-walled 

 oesophagus projects the papilla-like anterior portion 

 of the mid-gut. 



The Diverticula of the Oesophagus. From the 

 oesophagus two or three diverticula, similar in nature 

 to the oesophagus, extend backwards. Of these, one 

 is of great size, and usually contains gas bubbles. 



