I30 



mid -gut is the true digestive portion of the 

 tract. 



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 commissures. At first it is 

 narrow, but posteriorly becomes a large chamber 

 (the pumping organ). (Fig. 43A). 



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. Those 

 of the superior plate consist of two muscular masses, 

 taking their origin from the occiput. Those of 

 the lateral plates consist on each side of a single 

 large muscular mass arising from the lateral 

 portions of the head. 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 vv^hole forms a very powerful 

 suctorial apparatus. 



The Oesophagus. — Immediately beyond the 

 pumping 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 pro- 

 jects so as to form a valvular flap. 



