CONCLUSION 



THE animals described in this book as examples of the 

 different grades of organisation, belong to one or the other of 

 the two great divisions of the animal kingdom, the Protozoa 

 or the Metazoa. The Protozoa may be roughly described as 

 unicellular animals, but it has been pointed out sufficiently 

 clearly in the first volume that such a description is inexact, 

 and that it is impossible to make a sharp distinction between 

 the two divisions. There are Protozoa which can only be 

 described as multicellular, and although consistency forbids 

 the idea of a unicellular Metazoon, it must not be forgotten 

 that all Metazoa begin their existences in the unicellular con- 

 dition. But all adult Metazoa are multicellular, and the cells 

 of which their bodies are composed, show a tendency 

 to arrange themselves in definite layers ahout a central cavity 

 or system of cavities. In one of the great sub-divisions of the 

 Metazoa, the Ccelenterata, there is only one such cavity, the 

 gastrovascular cavity, serving for digestion and circulation 

 alike. This cavity is bounded by two cellular layers, known 

 as the ectoderm and endoderm, with a gelatinoid, structureless 

 layer between. The structural plan of a coelenterate animal 

 is, therefore, of the simplest kind, and, however complicated 

 the variations of the plan may seem in the different jelly-fishes, 

 corals, sea-anemones, and other polyps belonging to the group, 

 it is always easy to reduce the complexity to the fundamental 

 plan of a simple two-walled sac, with an opening at one end, 

 and the organs disposed radially with regard to that opening. 

 The other great division of the Metazoa, the Ccelomata, 

 comprises a vast and very varied assemblage of animals, which 

 have this one character in common, that the muscular, 

 circulatory, and reproductive organs are dissociated from 

 the external and internal epithelial layers i.e. from the 

 ectoderm and endoderm, and form an intermediate layer or 

 layers, collectively described as the mesoderm or mesoblast. 



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