ENTERIC CANAL 



371 



spoken of as the stomach, 

 which occupies a great part of 

 the interior of the head, and 

 is divided into a large ante- 

 rior division (a), 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 (/id) 

 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 connec- 

 tive-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 se- 

 cretes a layer of chitin, which 

 thus constitutes the innermost 

 layer of their cavities. It is 

 proved by development that 

 the mid-gut, which has no 

 chitinous lining, is the only 



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

 fish with part of the exoskeleton 

 removed to show the muscles. 

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

 extensor muscle ; ft. flexor muscle. (From Parker and Haswell'? Zoology.} 



B B 2 



