160 SUMMARY. 



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 mesoblastic structures. 



In all the higher Coeleuterata certain tissues become interposed 

 between the epiblast and hypoblast, which may be classified 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 epithelial 

 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 membrane, ^Equorea 

 Mitrocoma. Such layers when on the outer side of the membrane 

 separating epiblast and hypoblast are undoubtedly epiblastic in 

 origin, but in some cases amongst the Actinozoa they adjoin the 

 hypoblast, and are very probably derived from this layer. 



The origin of the gelatinous tissue is still involved in much 

 obscurity. 



It originates as a homogeneous layer between epiblast and hypo- 

 blast, which in the Hydromedusa? never becomes cellular though 

 traversed by elastic fibres. 



In the Acraspeda it contains anastomosing cells in the main 

 apparently (Glaus) derived from the hypoblast, and in the Ctenophora 

 it is richly supplied with muscular stellate cells for the most part 

 of epiblastic origin, though some are stated by Chun to come from 

 the hypoblast. On the whole it seems probable, that the gelatinous 

 tissue may be regarded as a product of both layers; and there are 

 some grounds for thinking that it is an immense development 

 of the membrane always interposed between the two primary layers. 

 It must however be borne in mind that a membrane, regarded by 

 the Hertwigs as the equivalent of the ordinary membrane between 

 the epiblast and hypoblast, can be usually demonstrated on both 

 surfaces of the gelatinous tissues in Medusas. The skeletogenous 

 layer of the Actinozoa is probably the morphological homologue 

 of the gelatinous tissue ; but the evidence we have is on the whole 

 in favour of the connective-tissue cells it contains being epiblastic 

 in origin. It gives rise to the skeleton of the Hexacoralla, to the 

 spicular skeleton of Alcyonium, the axial skeleton of Cora-Ilium, and 

 the skeleton of the Helioporidse and Tubiporidse. 



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

 treated of in the second part of this work. 



