XXIV ENTERIC CANAL 277 



dorsal line the parietal layer of coelomic epithelium becomes 

 deflected downwards, forming a two-layered membrane, the 

 dorsal mesentery; the two layers of this on reaching the 

 enteric canal diverge and pass one on either side of it, form- 

 ing the visceral layer of coelomic epithelium ; uniting again 

 below the canal, they are continued downwards as the ventral 

 mesentery, and on reaching the body-wall diverge once more 

 to join the parietal layer. Thus the mesenteries are simply 

 formed of a double layer of coelomic epithelium, continuous 

 on the one hand with the parietal and on the other with the 

 visceral layer of that membrane. 



Beside the mesenteries, the canal is supported by trans- 

 verse vertical partitions or septa (Fig. 67, a and b. Sept) which 

 extend right across the body-cavity, each being perforated by 

 the canal. The septa are regularly arranged and correspond 

 with the external grooves by which the body is divided into 

 metameres. Thus the transverse or metameric segmen- 

 tation affects the coelome as well as the body-wall. Each 

 septum is. composed of a sheet of muscle covered on both 

 sides with coelomic epithelium (b, Sept). 



Where the septa come in contact with the enteric canal, 

 the latter is more or less definitely constricted, so as to pre- 

 sent a beaded appearance (a and b) ; thus we have segmen- 

 tation of the canal as well as of the body-wall and coelome. 



The digestive canal, moreover, is not a simple tube of 

 even calibre throughout, but is divisible into four portions. 

 The first or pharynx {Ph) is very short, and can be pro- 

 truded during feeding ; the second, called the gullet or 

 oesophagus (6>«), is confined to the peristomium and is distin- 

 guished by its thick walls and comparatively great diameter ; 

 the third or intestine {Int) extends from the first metamere 

 to the last — i.e., from the segment immediately following 

 the peristomium to that immediately preceding the anal 



