THE SUB-KINGDOM C(ELENTERATA. 17 



begin to ensue ; but it is sufficient here to state, 

 that, eventually, the principal nervous and vascular 

 trunks are found to occupy opposite aspects of 

 the body, the axis of which is traversed by the 

 alimentary canal. Thus in every Vertebrate, An- 

 nulose, and Molluscous animal it is possible to re- 

 cognise two distinct regions, a nervous, or * neural,' 

 and a vascular, or ' haemal.' 



In the Coelenterata, on the other hand, no dis- 

 tinction between neural and haemal regions can 

 be noticed. Furthermore, whatever outward com- 

 plexity the organism may present, all its parts 

 are found on examination to be readily resolvable 

 into the two layers previously referred to as ecto- 

 derm and endoderm. These correspond, both in 

 structure and mode of growth, with the primitive 

 layers of the germ in the higher animals, so that 

 a general analogy may be traced between the 

 permanent condition of the Ccelenterata and a 

 well-marked embryonic stage in the Mollusca, 

 Anmdosa, and Vertebrata. 



This is especially the case with the Hydrozoa, 

 in which class a body-wall, composed of ectodermal 

 and endodermal layers, invests the simple undivided 

 cavity which constitutes the whole interior of the 

 animal {fig. 2, e). The alimentary and somatic 

 cavities are, therefore, in these beings identical, 

 though, among many members of the group, 

 certain parts of the organism are more imme- 

 diately concerned in the performance of the di- 

 gestive function. 



But in the Actinozoa an oral fold of the blas- 

 toderm grows inwards to form a distinct digestive 

 sac ; thus, as it were, suspended, in the general 

 cavity of the body, with which it communicates 

 c 



