V CCELOME 325 



animal as typified by the earthworm, which may be sum- 

 marised as follows. It consists of two tubes formed of 

 epithelial cells, one within and parallel to the other, the 

 two being continuous at either end of the body, where the 

 inner tube (enteric epithelium) is in free communication 

 with the exterior ; the outer tube (deric epithelium) is lined 

 by a layer of connective tissue and muscle-fibres, within 

 which is a thin peritoneum lined by ccslomic epithelium, the 

 three together forming the body-wall ; the inner tube 

 (enteric epithelium) is covered externally by a layer of 

 muscle-fibres and connective tissue and a thin peri- 

 toneum covered by coelomic epithelium, which form with 

 it the enteric canal ; lastly, the body-wall and enteric canal 

 are separated by a considerable space, the ccelome. 



The enteric canal is not, as might be supposed from the 

 foregoing description, connected with the body-wall only at 

 the mouth and anus, but is supported in a peculiar way. 

 There is no dorsal mesentery as in the frog (p. 27), but a 

 series of transverse vertical partitions or septa (Fig. 79) 

 extend right across the body-cavity, each being perforated 

 by the canal. The septa are regularly arranged and cor- 

 respond in position with the external grooves by 

 which the body is divided into metameres. Thus 

 the transverse or metameric segmentation affects the 

 ccelome as well as the body-wall, the former being divided 

 up into a series of chambers, which, however, communicate 

 \with one another ventrally, where the septa are incomplete. 

 Each septum is composed of a sheet of connective tissue 

 and muscle-fibres, and is covered on both sides by ccelomic 

 epithelium. The coelome'communicates with the exterior by 

 a series of dorsal pores situated in the grooves between all 

 the segments except about the first ten. 



