THE ORGANISM AS A MECHANISM 77 



Peridinians, for instance, which have much of the 

 structure of tiie animal (though cellulose is present 

 in their skeleton), which possess motile organs, but 

 which also possess a photo-synthetic apparatus, and 

 exhibit the typical plant mode of nutrition. Further, 

 there are symbiotic partnerships, that is, associations 

 of plant and animal in one " individual " form (as, 

 for instance, among the lower worms, Echinoderms, 

 polyzoa, molluscs, and other groups of animals). In 

 these cases green algal cells, capable of forming starch 

 from carbon dioxide and water under the influence of 

 light, become intercalated among the tissues of the 

 animal. We find, also, that with regard to some 

 fundamental characters, plant and animal display 

 close similarities : the structure of the cell, for ex- 

 ample, and the highly special mode of conjugation 

 of the germ-nuclei in sexual reproduction. We must 

 regard all the distinctive characters of the plant as 

 represented in the animal and vice versa. Wh}'' they 

 have become specialised in different directions is a 

 question that we discuss later. 



The organism, then, in so far as we regard it 

 as a physico-chemical mechanism, as the theatre of 

 energetic happenings, exhibits the following general 

 characters : — 



(i) It slowly accumulates available energy in the 



form of chemical compounds of high potential, 



work being done upon it. 



(2) It liberates this energy in relatively rapid, con- 



trolled, " explosive reactions," transforming 

 into movements carried out by a sensori- 

 motor system of parts, work being done by it. 



(3) In all these transformations the amount of 



energy which is dissipated is relatively small, 

 and tends to vanish. 



