324 



THE EARTHWORM 



Mesoderm . 

 (rudimentary) ' 



and with it forms the enteric canal. The coelome separates 

 the parietal and visceral layers from one another, and is 

 lined throughout by coelomic epithelium. 



The relation between the diploblastic polype and the 

 triploblastic worm may therefore be expressed in a tabular 

 form as follows — ■ 



Hydroid Earthworm} 



Cuticle Cuticle. 



Ectoderm Deric epithelium or epiderm. 



I Connective tissue and muscle- 

 Parietal fibres. 

 layer 1 Peritoneum with its coelomic 

 y epithelium (parietal layer). 



(Peritoneum with its coelomic 

 epithelium (visceral layer). 

 Connective tissue and muscle- 

 fibres. 



Endoderm Enteric epithelium. 



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 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 tripoblastic 



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

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

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



