332 



THE EAKTHWORM 



CHAP. 



coelome separates the parietal and visceral layers from 

 one another, and is lined throughout by crelomic 

 epithelium. 



The relation between the diploblastic polype and the 

 triploblastic worm may therefore be expressed in a 

 tabular form as follows : 



Hydroid. Earthworm. 1 



Cuticle Cuticle. 



Ectoderm Deric epithelium or epiderm. 



( Connective-tissue and mus- 

 Parietal cle-fibres. 



layer 1 Peritoneum with its ccelomic 

 I epithelium (parietal layer). 



! Peritoneum with its ccelomic 

 epithelium (visceral layer). 

 Connective-tissue and mus- 

 cle-fibres. 

 Endoderm .... Enteric epithelium. 



Mesoderm . 

 (rudimen- 

 tary) 



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 epithelium, and the study of development shows 

 that the cells lining both the anterior and posterior ends 

 of the canal are ectodermal (compare pp. 204 and 207). 

 For this reason the terms deric and enteric epithelium 

 are not mere synonyms of ectoderm and endoderm 

 respectively. 



It is important that you should, before reading further, 

 understand clearly the general composition of a triplo- 



1 It will be seen that the relations of these layers in the earth- 

 worm and frog are similar, except that in the latter the cuticle is 

 wanting (compare Figs. 5, 38, and 39). 



