THE ANATOMY OF PBNTASTOMUM TEEETIUSOULUM. 19 



separated by irregular intervals from each other, increase in 

 length and lie closely side by side. They form a small swell- 

 ing under the middle part of the tooth-like organ, but 

 at the upper termination of the latter the layer becomes more 

 than one cell deep ('16 mm. in thickness), and, together with 

 the special differentiation above alluded to (fig. 32, P. E}), 

 they give rise to a cushion or pad in the posterior wall of the 

 pharynx where the latter turns sharply backwards. It is to 

 this smaller upper part of the pharynx that the most important 

 muscles are attached. From the anterior and side walls arise 

 the bands passing forwards and downwards to the cuticular 

 ingrowths from the side of the papilla ; from the posterior wall 

 arise fibres passing downwards and backwards into connection 

 with the longitudinal layer of the body. Another special band 

 from the latter is inserted into the cuticle just at the angle of 

 the mouth {M. 7). 



There are thus in connection with the mouth and pharynx 

 live main sets of muscles : — (1) Muscles of the papilla. (2) 

 Muscles from the anterior and side walls of the upper part of 

 the pharynx. (3) Muscles forward from the papilla to the 

 head. (4) Muscles from the posterior surface of the upper 

 part of the pharynx. (5) Muscles inserted into the angle of 

 the mouth posteriorly. 



The third series assist the first two, and the united action 

 of all five will be, as can be seen on reference to the figure, 

 (1) to distend the cavity of the pharynx ; (2) to pull the 

 pharynx downwards ; (3) to pull the papilla backwards and 

 slightly upwards ; (4) to distend the mouth opening. In this 

 way the pharynx, oral papilla, and tooth-like organ serve as 

 sucking, and the latter two as rasping structures. 



(3) (Esophagus. — This is some four times as long as the 

 pharynx, from which it runs obliquely backwards and up- 

 wards till it reaches the ventral wall of the stomach (figs. 27 

 and 50). The walls throughout its whole course are formed 

 of a layer of columnar epithelium directly continuous with 

 that of the pharynx, and lined internally by a thin but dis- 

 tinct cuticle. External to the epithelium lies the muscle 



