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THE SUB-KINGDOM CCELENTERATA. 



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 

 a ^ 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- 



Qa l plexity the organism may present, all its parts 



t i are found on examination to be readily resolvable 



into the two layers previously referred to as ecto- 



, e * 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 Goelenterata and a 



pfaon well-marked embryonic stage in the Mollusca. 



Annulosa, and Vertebrata. 



Eyd 



in which class a body-wall, composed of ectodermal 

 er as and endodermal layers, invests the simple undivided 



* 



?r fin 



tnths 



cavity which constitutes the whole interior of the 

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

 cavities are, therefore, in these beings identical, 

 ■media though, among many members of the 



group, 

 certain parts of the organism are more imme- 

 f or , diately concerned in the performance of the di- 

 =K jj a tt - 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 



