vi CCELOME 333 



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 ccelomic 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 peritoneum 

 covered by ccelomic 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 (Figs. 82 and 83) 

 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 trans- 

 verse 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 (Fig. 83, 

 n.a). Each septum is composed of a sheet of connective- 

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

 coelomic epithelium. The ccelome communicates with the 

 exterior by a series of dorsal pores situated in the grooves 

 between all the segments except about the first ten. 



