n METABOLISM 249 



of all the ingesta of the organism (carbon dioxide PLUS 

 mineral salts plus respiratory oxygen) they would be 

 found to contain less oxygen than the egesta (oxygen 

 from decomposition of carbon dioxide plus water, 

 excreted carbon dioxide, and nitrogenous waste) ; so 

 that the nutritive process in Sphaerella is, as a whole, a 

 process of deoxidation. In Amoeba, on the other hand, 

 the ingesta (food plus respiratory oxygen) contain more 

 oxygen than the egesta (faeces plus carbon dioxide, 

 water, and nitrogenous excreta), the nutritive process 

 being therefore on the whole one of oxidation. This 

 difference is, speaking broadly, characteristic of plants 

 and animals generally ; animals, as a rule, take in more 

 free oxygen than they give out, while green plants 

 always give out more than they take in. 



Destructive metabolism is, however, manifested not 

 only in the formation of waste-products, but in that of 

 substances simpler than protoplasm which remain an 

 integral part of the organism, viz., cellulose and starch. 

 The cell-wall is probably formed by the conversion of a 

 thin superficial layer of protoplasm into cellulose, the 

 cyst attaining its final thickness by frequent repetition of 

 the process. The starch of the pyrenoids is apparently 

 formed by a similar process of decomposition or de- 

 structive metabolism of protoplasm. 



\Ye see then that destructive metabolism may result 

 in the formation of (a) waste products and (b) plastic 

 products, the former being got rid of as of no further use, 

 while the latter remain an integral part of the organism. 



Let us now turn once more to the movements of 

 Sphaerella, and consider in some detail the manner of 

 their performance. 



Each flagellum (Fig. 68, A, fl} is a thread of proto- 

 plasm of uniform diameter except at its distal or free 



