SECT, vii THE ALIMENTARY CANAL 113 



whole class of the Crustacea as one of their most 

 constant characteristics. It is not easy to imagine a 

 simpler or more likely explanation of this extra- 

 ordinary bend in the intestinal tract than that here 

 given ; and if it is true, its importance for the purposes 

 of classification is at once evident. We shall return 

 to this subject in the section on the relations of Apus 

 to Limulus and to the Trilobites. 



The oesophagus itself corresponds with that of the 

 original Annelid, which was probably provided with 

 a protrusible pharynx. The loss of this proboscis 

 would naturally follow on the adoption of a browsing 

 manner of life, and the gradual adaptation of the para- 

 podia as instruments for pushing food into the mouth. 

 The oesophagus is very muscular, and is provided 

 with muscle bands radiating forwards into the fore- 

 head, and backwards into the sinewy mass already 

 described. These bands serve to dilate it, while its 

 powerful circular muscles close it ; when closed it is 

 thrown into folds. The dilators may perhaps be the 

 remains of the retractors and extensors of the 

 pharynx. The oesophagus is lined by a chitinous 

 intima and provided with setae which project upwards 

 so as to form a fish-trap apparatus. The paired glands 

 which open on the under lip close to the mouth (see 

 Fig. 29) will be described in the section (IX.) on the 

 excretory and other glands. 



The oesophagus is in Apus comparatively simple, 

 but it is easy to see how a part of such a muscular 

 apparatus, with its chitinous intima folded, and thrown 

 into strong movement by every act of swallowing, 



I 



