8 JEAN DAWSON. 



The position of these muscles explains the difference between the 

 size of the pharyngeus muscle viewed from the dorsal side (Fig. 

 4) and the actual size of the pharyngeal cavity (Figs. I and 5). 



The outer supporting walls of the pharyngeal cavity outside of 

 the pharyngeal muscle are : on the ventral side, the large tongue 

 muscle ; on the dorsal side, the ethmoid cartilage ; on the ante- 

 rior one half of the lateral walls, the basilaris muscle which ex- 

 pands and contracts the anterior half of pharyngeal cavity ; lying 

 dorsal to the basilaris muscles are found the large salivary glands 

 which when the muscles act cause, according to Fiirbringer 

 (1875), a great flow of saliva. 



The posterior pharyngeus is a strong bundle of muscular fibers 

 arching over the mouth of the oesophagus between the walls of 

 the nasal canal and that of the oesophagus (Fig. i, k, and Fig. 4, /)., 

 This muscle lies just posterior to the pharyngeus muscle and 

 when contracted closes off the oesophagus completely from the 

 pharynx. When food reaches the posterior part of the pharynx, 

 this muscle must relax to allow the food to pass on its way to 

 the intestine. 



The mucous membrane on the dorsal wall of the pharynx is con- 

 tinuous with that on the dorsal wall of the oesophagus, while that on 

 the ventral wall is continuous with that on the ventral wall of the 

 water tube. The ventral wall of the oesophagus and the dorsal 

 wall of the water tube begin where the pharynx ends and con- 

 tinue caudad in close contact, parallel with the long axis of the 

 body, to where the water tube ends, just anterior to the pericar- 

 dium (Fig. i). At the extreme anterior edge of the united 

 walls of the pharynx and water tube there projects forward a 

 pair of jaws which, on account of their connection with the velar 

 valves, may be called velar jaws (Fig. i,_/; Fig. 5,7 ; and Fig. 

 6, _/). They extend into the posterior part of the pharyngeal 

 cavity and look like the jaws of a beetle. So close is the re- 

 semblance that at first sight one easily imagines that the animal 

 has by some means swallowed a beetle, the jaws of which are 

 lying in the pharynx. In Fig. 4, these jaws would lie beneath 

 the triangular raphe (d). 



The jaws are smooth and glossy and are covered with mucous 

 membrane. Each is thick at the base where it unites with the 



