II CILIARY MOVEMENT 33 



istic of })lunts 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 (see p. 14). 

 The starch of the pyrenoids is apparently formed by a similar 

 process of decomposition or destructive metabolism of pro- 

 toplasm, growth taking place, in both instances, by accretion 

 and not by intussusception. 



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 once more to the movements of Hoemato- 

 coccus, and consider in some detail the manner of their 

 performance. 



Each flagellum (Fig. 3, .\, fl) is a thread of protoplasm of 

 uniform diameter except at its distal or free end where it 

 tapers to a point. The lashing movements are brought 

 about by the flagellum bending successively in different 

 directions ; for instance, if in Fig. 3 f, abc represents it in 

 the position of rest, abc' will show the form assumed when 

 it is deflected to the left, and abc" when the bending is 

 towards the right. In the position abc the two sides ab, ac 

 are obviously equal to one another, but in the fle.xed 

 positions it is equally obvious that the concave sides ac\ be" 

 are shorter than the convex sides bc\ ac" ; in other words, as 



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