XXIV DIPLOBLASTIC AND TRIPLOBLASTIC FORMS 275 



But it will be remembered that in polypes there is some- 

 times found a layer of separate muscle-fibres between the 

 ectoderm and the mesogloea, and it was pointed out (p. 236) 

 that such fibres represented a rudimentary intermediate cell- 

 layer or mesoderm. AVe may therefore consider the muscular 

 layer and the coelomic epithelium of Polygordius as meso- 

 derm, and we may say that in this animal the mesoderm is 

 divisible into an outer or somatic layer, consisting of the 

 muscle-plates and the parietal layer of ccelomic epithelium, 

 and an inner or splanchnic layer, consisting of the visceral 

 layer of coelomic epithelium.^ 



The somatic layer is in contact with the ectoderm or deric 

 epithelium, and with it forms the body-wall ; the splanchnic 

 layer is in contact with the endoderm or enteric epithelium 

 and with it forms the enteric canal. The ccelome separates 

 the somatic and splanchnic 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 Polygordius. 



Cuticle Cuticle. 



Ectoderm .... Deric epithelium or epidermis. 



o .• , Muscle-plates. 

 Somatic j ^ 



,, J 1 ,' Coelomic epithelium 



Mesoderm . .j layer \ , . ,, . 



(rudimentary)! _ _ ' ^P^""^^^ ^^>'"'")- 



I Splanchnic j Ccolomic epithelium 



layer ) (visceral layer). 



Endoderm Enteric epithelium. 



^ In the majority of the higher animals there is a layer of muscle 

 between the enteric and coelomic epithelia : in such cases the body-wall 

 and enteric canal consist of the same layers but in reverse order, the 

 coelomic epithelium being internal in the one, external in the other. 



T 2 



