90 



As suggested by Huxley and Mocquard the function of 

 these valves may be partly to prevent the waste matter 

 from passing back into the fore-gut, but Cuenot has 

 claimed that the valves are also used, as described above, 

 for carrying the hard waste pieces directly into the hind- 

 gut. The mid-gut is not lined with chitin, and con- 

 sequently the sharp pieces present amongst the food in 

 the alimentary canal would be liable to tear the walls of 

 the mid-gut. 



The soft parts of the aliments are passed through the 

 narrow ventral portion of the pyloric region, where they 

 are sieved by the setae stretching across the lumen, and 

 near the posterior region of this region the food comes 

 into contact with the secretion from the digestive glands. 

 The latter open into the ventro-lateral wall of the 

 mid-gut immediately behind the ampullae, and the 

 digestive fluid flows forward and mixes with the food in 

 the ventral part of the pyloric region. 



In the cardiac and pyloric regions we have essentially 

 the same histological arrangements as in the oesophagus. 

 The epidermis consists of columnar epithelium of much 

 less length than the cells of the oesophageal epidermis. 

 The chitinous layer is very thin except in the regions of 

 the ossicles of the gastric mill, which are merely thick 

 portions of the chitin which have become strongly 

 calcified. The basement membrane 'is well marked and 

 the connective tissue is a very thin layer. Embedded in 

 this layer are very thin bands of circular and longitudinal 

 muscles. 



Mid-gut (or Mesenteron) (PI. V, fig. 34, m. g.}. 



The mid-gut is an extremely short portion of the 

 alimentary canal, being only about 10 mm. long in a 

 full-grown crab. This is the only part of the alimentary 



