vi CCELOME 333 



blastic animal as typified by the earthworm, which may 

 be summarised 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 tlie inner tube (enteric epithelium) is in free com- 

 munication with the exterior ; the outer tube (deric 

 epithelium) is lined by a layer of connectfve-tissue and 

 muscle-fibres, within which is a thin peritoneum lined by 

 coelomic 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 witn 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 par- 

 titions or septa (Figs. 82 and 83) extends right across the 

 body-cavity, each being perforated by the canal. The 

 septa are regularly arranged and correspond 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 (Fig. 83, n. a). Each 

 septum is composed of a sheet of connective-tissue and 

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

 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. 



