VII 



ENTERIC CANAL 



371 



which occupies a great part 

 of the interior of the head, 

 and is divided into a large 

 anterior division (cs) and a 

 smaller posterior division 

 (ps) ; the latter passes into 

 the intestine, which consists 

 of a narrow and very short 

 mid-gut or mesenteron (md), 

 from which a somewhat 

 wider hind-gut hd) extends 

 to the anus (an), situated 

 on the ventral surface of 

 the telson. The gullet and 

 gizzard together constitute 

 the fore -gut. 



The outer layer of the en- 

 teric canal consists of con- 

 nective-tissue containing 

 striped muscular fibres ; 

 within this is a single layer 

 of columnar epithelial cells, 

 none of them glandular. In 

 the fore- and hind-gut the 

 epithelium secretes a layer 

 of chitin, which thus consti- 

 tutes the innermost layer of 

 their cavities. It is proved by 

 development that the mid- 

 gut, which has no chitinous 

 lining, is the only part of 



FIG. 92. A leg of the Fresh-water Cray- 

 fish with part of the exoskeleton 

 removed to show the muscles. ( X 5 . ) 

 en. 2-en. 5, segments of endopodite ; h. hinges ; art. m. articular membrane; ext. 

 extensor muscle ; fl. flexor muscle. (From Parker and Haswell's Zoology.) 



B B 2 



