XX INTRODUCTIOX. 



we have liere (as Dr. Farrc has suggested) a special 

 muscular apparatus, by which the action of this organ is 

 regulated. The pharyngeal cavity, in such cases, must be 

 regarded as playing an important subsidiary part in con- 

 nexion with the nutritive system. Here the particles of 

 food gather in the first instance, and, it would seem, are 

 submitted to some kind of selective process : when a 

 sufficient amount of suitable material has collected, the 

 constriction at the base of the canal is relaxed, and the 

 food passes rapidly, impelled by the pharyngeal spasm, 

 and subsequently by the peristaltic movements of the oeso- 

 phagus, to the stomach. In other cases, the pharynx is 

 not so clearly defined, and the oral region of the eso- 

 phagus is chiefly distinguished from the rest by its cili- 

 ated lining. 



The oesophagus varies much in length in different spe- 

 cies ■^: it is usually a straight and slender tube, which 

 leads directly into the digestive sac. In the coiirse of this 

 tube is placed a valve (the cardia), marking the entrance 

 to the stomach, and consisting of a conical perforated 

 projection, with the free extremity directed downwards^ 

 which opens for the passage of the food, but is otherwise 

 closed, and forms a barrier preventing its return into the 

 oesophagus (Woodcuts, figs. vi. & ix, c). 



The position of the cardia varies in different species ; it 

 is sometimes placed high up in the oesophagus, and some- 

 times much lower down and close to the true stomach. 

 In Beania mirabilis (Woodcut, fig. vii.) I can detect no 

 separation between the stomach and the oesophageal re- 

 gion but the constriction which closes the funnel-shaped 



* Compare Woodcuts, figs. vii. and vi. In the former {Bccniia Qnirahilia) 

 it is of remarkable length; in the latter {Buf/ula phtiiioKi) it if almost rudi- 

 nieiitary. 



