ii METABOLISM 249 



dioxide is evolved ; and if an analysis were made of all 

 the ingesta of the organism (carbon dioxide plus mineral 

 salts plus respiratory oxygen) they would be found to con- 

 tain less oxygen than the egesta (oxgyen 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 respi- 

 ratory 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, character- 

 istic 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. 



But destructive metabolism is 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 destructive metabolism of pro- 

 toplasm. 



We 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 <mru more to the movements of 

 Sphserella, and consider in some detail the manner of their 

 performance. 



