118 
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. 
