CCELENTERATA. U9 



accepted, the absolutely inadmissible hypothesis of a double origin 

 for the Coelenterata would have to be adopted. 



Two questions arise from these considerations : 



(1) Which is the primitive, delamination or invagination ? 



(2) How is the one of these to be derived from the other ? 

 There is' a great deal to be said in favour of both these views ; 



but it will be convenient to defer all discussion of the question 

 to the general chapter on the formation of the layers throughout 

 the animal kingdom. 



The hypoblast cells are often filled with yolk material, and 

 secondary modifications are thus produced in the development. The 

 most important examples of such modifications are found in the 

 Siphonophora and Ctenophora. 



In the simplest forms amongst the Hydrozoa there is no trace 

 of a third layer or mesoblast. The epiblast is typically formed, 

 as was first shewn by Kleiuenberg, of an epithelial layer and a 

 subepithelial interstitial layer of cells. The cells of the former are 

 frequently produced into muscular or nervous tails, and those of 

 the latter give rise to the thread-cells and generative organs and 

 in some cases to muscles 1 . In many cases, amongst all the Coelenterate 

 groups, and constantly amongst the Ctenophora the epiblast is simpli- 

 fied and reduced to a single layer. The hypoblast undergoes in most 

 cases no such differentiation but simply forms a glandular layer lining 

 the gastric chamber and its prolongations into the tentacles ; but in 

 the Actinozoa it appears to give rise to muscles, and strong evidence 

 has been brought forward to shew that in some groups it gives rise 

 to the generative organs. 



Between the epiblast and hypoblast a structureless lamella appears 

 always to be interposed. 



In many Coelenterata further differentiations of the epiblast are 

 present. In many forms the layer gives rise to a hard external 

 skeleton. This is most widely spread amongst the Hydrozoa, where 

 in the majority of cases it takes the form of the horny perisarc, 

 and in the Hydrocoralla (Millepora and Stylasteridaa) of a hard 

 calcareous skeleton. The skeleton in these forms, though closely 

 resembling the mesoblastic skeleton of the Actinozoa, has been shewn 

 by Moseley (164) to be epiblastic. 



In the Actinozoa an epiblastic skeleton is exceptional, and 

 according to most authorities absent. Quite recently however Koch 

 (167) has found that the axial branched skeleton of most of the Gorgo- 

 nidse, viz. the Gorgoninaa and Isidinae, is separated from the ccenosarc 

 by an epithelium, which he believes to be epiblastic, and to which 

 no doubt the axial skeleton owes its origin. A similar epithelium 

 surrounds the axis of the Pennatulidae. 



In the Medusas the epiblast also gives rise to a central nervous 



1 The questions relating to the generative organs of the Cuelenterata are dealt with 

 in the second part of this work. 



