THE MOST PRIMITIVE METAZOA 77 



having the morphological value of the epidermis of the higher 

 animals, and the latter that of the epithelium of the alimentary 

 canal. Between these two layers, a third layer — the mesoderm — 

 which represents the structures which lie between the epidermis 

 and the epithelium in more complex animals, may be developed, 

 and sometimes attains great thickness, but it is a secondary and, 

 in the lower Hydrozoa, inconspicuous production. Notwithstand- 

 ing the extreme variety of form exhibited by the Hydrozoa and the 

 multiplicity and complexity of the organs which some of them 

 possess, they never lose the traces of this primitive simplicity of 

 organisation and it is but rarely that it is even disguised to any 

 considerable extent. ... In the fundamental composition of the 

 body of an ectoderm and an endoderm, with a more or less largely 

 developed mesoderm, and the abundance of thread cells, the 



Actinozoa agree with the Hydrozoa There is a certain similarity 



between the adult state of the lower animals and the embryonic 

 conditions of the higher organisations. For it is well known that, 

 in a very early state, even of the highest animals, it is a more or less 

 complete sac, whose thin wall is divisible into two membranes, 

 an inner and an outer. . . . There is a very real and genuine 

 analogy between the adult Hydrozoon and the embryonic vertebrate 

 animal, but I need hardly say it by no means justifies the assump- 

 tion that the Hydrozoa are in any sense ' arrested developments ' 

 of higher organisms." 



From the above account by Huxley two points are clear. Firstly 

 he thought that the resemblance between the embryonic develop- 

 ment of the higher animals and the organisation of the coelenter- 

 ates into two main layers of importance as indicating the primitive- 

 ness of the coelenterates. Secondly Huxley realised that mesoderm, 

 or its precursor, did occur in the coelenterates and thought that 

 there was an increase in the thickness and complexity of the 

 mesoderm in the higher coelenterates. In effect he assumed that 

 simple Hydrozoa such as Hydra and Tubularia were more 

 primitive than the members of the Actinozoa. 



There are now two questions that should be considered. The 

 first is what are the simple and complex characters of the 

 coelenterates? From a study of these it should be possible to 

 assess how near the coelenterates are to the basic metazoans. The 

 second problem concerns the relationship of the Hydrozoa, 



