CCELENTERATA. l8l 



In many Coelenterata further differentiations of the epiblast 

 are present. In many forms the layer gives rise to a hard exter- 

 nal 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 Stylasteridae) 

 of a hard calcareous skeleton. The skeleton in these forms, 

 though closely resembling the mesoblastic skeleton of the Actin- 

 ozoa, 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 Gorgonidae, viz. the Gorgoninae and Isidinse, 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 Penna- 

 tulidae. 



In the Medusas the epiblast also gives rise to a central 

 nervous system, which however continues to form a constituent 

 part of the layer, and to the organs of special sense 1 . 



A special differentiation of the hypoblast is found in the 

 solid axis of the tentacles. This axis replaces the gastric 

 prolongation found in many forms, and the cells composing 

 it differentiate themselves into a chorda-like tissue, which has 

 a skeletal function, and is no longer connected with nutrition. 

 This axis is placed by many morphologists amongst the meso- 

 blastic structures. 



In all the higher Ccelenterata certain tissues become inter- 

 posed between the epiblast and hypoblast, which may be classi- 

 fied together as the mesoblast. 



The most important of these are 



(1) The various distinct muscular layers. 



(2) The gelatinous tissue of the Medusae and Ctenophora. 



(3) The skeletogenous tissue of the Actinozoa. 



In most cases the muscular fibres are connected with epithe- 

 lial cells, but in certain forms amongst the Medusae and in the 

 majority if not all the Actinozoa they constitute a distinct layer, 

 sometimes separated from the epiblast by a structureless mem- 



1 The differentiation of the nervous and muscular systems in the Hydrozoa is 

 treated of in the second part of this work. 



