xxv GENERAL STRUCTURE 279 



Strictly speaking, this comparison does not hold good of 

 the anterior and posterior ends of the worm : at both mouth 

 and anus the deric passes insensibly into the enteric epithe- 

 lium, and the study of development shows (p. 298) that the 

 cells lining both the anterior and posterior ends of the canal 

 are, as indicated in the diagram (Fig. 68, B), ectodermal. For 

 this reason the terms deric and enteric epithelium are not 

 mere synonyms of ectoderm and endoderm respectively. 



It is important that the student should, before reading 

 further, understand clearly the general composition of a 

 triploblastic animal as typified by Polygordius, 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 the 

 inner tube (enteric epithelium) is in free communication 

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

 by a layer of muscle-plates within which is a thin layer of 

 ccelomic epithelium, the three together forming the body- 

 wall; the inner tube (enteric epithelium) is covered ex- 

 ternally by a layer of ccelomic epithelium which forms 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 and 

 somewhat complicated way. In the first place there are 

 thin vertical plates, the dorsal and ventral mesenteries (Fig. 

 68, A and c, D. Mes, V, Mes}, which extend longitudinally 

 from the dorsal and ventral surfaces of the canal to the body 

 wall, dividing the ccelome into right and left halves. The 

 structure of the mesenteries is seen in a transverse section 

 (Fig. 68, c, and Fig. 71, A) which shows that at the middle 



