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THE AMERICAN NATURALIST [Vol. XLII 



The arrangement of the cells in the body of a multi- 

 cellular animal is not, as in most plants, of a more or 

 less promiscuous kind, but conforms to certain funda- 

 mental principles. Among these probably the most im- 

 portant is one that depends upon the fact that animals 

 assimilate solid food. To carry out this operation they 

 possess almost universally a digestive cavity into which 

 this solid food is carried and there rendered soluble. The 

 only multicellular animals that do not possess a digestive 

 cavity are certain parasites, like the tapeworm, whose 

 modes of life are such as to make digestive organs super- 

 fluous. In the simpler multicellular forms, the coral ani- 

 mals, etc., the whole animal is sac-like in structure and 

 may be briefly described as an animated stomach. Not 

 only is this state characteristic of such primitive forms, but 

 in the development of almost every multicellular animal 

 known, the first organ to be formed is the digestive organ. 

 This is brought about in that the cells, which are destined 

 for the future animal's body, become arranged in the 

 form of a two-layered sac in which the inner layer or 

 entoderm bounds the digestive space and the outer one 

 or ectoderm acts as a protective covering. Both layers 

 develop a certain amount of muscular tissue and this is 

 brought into action through external stimuli that from 

 the nature of the case are received usually by the ecto- 

 derm. Hence this layer becomes the seat of those changes 

 which in the higher animals eventually shape themselves 

 in the sensory and nervous organization of these forms, 

 while the entoderm is concerned with the digestive func- 

 tions. Between these two layers there develops in all 

 the higher multicellular animals a third layer, the meso- 

 derm, which, as has been intimated, is primarily muscular 

 in character but may also give rise to the internal skele- 

 ton, circulatory system, and other related sets of organs. 

 Thus the body of a multicellular animal is composed of 

 definite layers or masses of cells, chiefly three in number, 

 with which special functions or modes of activity have 

 become firmly associated. 



