10 THE SUB-KINGDOM CCELENTEKATA. 



tions in external aspect presented by the Protozoa, 

 does not find his astonishment lessen when he 

 begins to contemplate the manifold endowments 

 of which each definable form is, as it were, the 

 index, and perceives the fundamental sameness of 

 organisation on which these complexities are based. 

 All this, however, reflection should have led him 

 to expect. For the body of every Vertebrate 

 animal was once of as simple a structure as that 

 of the Protozoon, and might even be said to cor- 

 respond with it. But the life-history of the former 

 plainly shows what it is capable of becoming. Some 

 would add that the vital nature of the two organisms 

 is less dissimilar than morphology and develop- 

 ment would seem to indicate, and that those ener- 

 gies, which the lower animals spend so rapidly in 

 acquiring the many outward modifications by 

 which they are soon distinguished, might, if duly 

 husbanded, and turned in another direction, give 

 rise to very different structural products. Such a 

 speculation is not wholly unworthy of mention. 

 At present its discussion would lead us too far 

 into the wide region of conjecture. 



Turning now to the sub-kingdom Coelenterata, 

 the members of this group are at once seen to dif- 

 fer widely from the Protozoa, in that their body- 

 substance resolves itself into the two layers already 

 mentioned under the names of ectoderm and en- 

 doderm ; the former serving the purpose of an 

 integument, the latter lining the large internal 

 cavity constantly present. 



These layers are very similar, though not iden- 

 tical, in structure. Both consist of a number of 

 vesicular bodies, or * endoplasts,' embedded in a 

 h omogeneous matrix, or ' periplast.' The endoplasts 



