CLASSIFICATION. 391 



We saw (§56), that the broadest division among the 

 functions is the division into " tlie accumulation of energy 

 (latent in food) ; the expenditure of energy (latent in the 

 tissues and certain matters absorbed by them) ; and the 

 transfer of energy (latent in the prepared nutriment or blood) 

 from the parts which accumulate to the parts which expend." 

 jSTow in the lowest animals, united under the general name 

 Protozoa, there is either no separation of the parts performing 

 these functions or very indistinct separation: in the Rhizo- 

 poda, all parts are alike accumulators of energy, expenders of 

 energy and transferers of energy; and though in the higher 

 members of the group, the Infusoria, there are some speciali- 

 zations corresponding to these functions, yet there are no 

 distinct tissues appropriated to them. Similarly when we 

 pass from simple types to compound types — from Protozoa to 

 Metazoa. The animals known as Coelenterata are charac- 

 terized in common by the possession of a part which accumu- 

 lates energy more or less marked off from the part which 

 does not accumulate energy, but only expends it; and the 

 Ilydrozoa and Actinozoa, which are sub-divisions of the 

 Coelenterata, are contrasted in this, that in the second these 

 parts are much more differentiated from one another, as well 

 as more complicated. Besides a completer differentiation of 

 the organs respectively devoted to the accumulation of 

 energy and the expenditure of energy, animals next above 

 the Ccdenterata possess rude appliances for the transfer of 

 energy: the peri-visceral sac, or closed cavity between the 

 intestine and the walls of the body, serves as a reservoir of 

 absorbed nutriment, from which the surrounding tissues take 

 up the materials they need. And then out of this sac 

 originates a more efficient appliance for the transfer of ener- 

 gy : the more highly-organized animals, belonging to 

 whichever sub-kingdom, all of them possess definitely-con- 

 structed channels for distributing the matters containing 

 energy. In all of them, too, the function of expenditure is 

 divided between a directive apparatus and an executive 

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