METABOLISM OF ANIMALS 



41 



grow and repeat the process in about ten hours so that, as 

 in the case of Sphaerella, within a few 

 days the original Paramecium has divided 

 its individuality, so to speak, among a 

 multitude of descendants. (Fig. 11.) 



This process of multiplying by dividing 

 can go on indefinitely under optimum en- 

 vironmental conditions. But periodically 

 Paramecium undergoes an internal nuclear 

 reorganization process (ENDOMIXIS). Also 

 now and then individuals temporarily fuse 

 in pairs and interchange nuclear material 

 (CONJUGATION) an expression of the 

 same fundamental sex phenomenon which 

 is exhibited in Sphaerella. (Figs. 12, 130, 

 131.) 



FIG 



Parame- 



B. METABOLISM IN PARAMECIUM 



cium aurelia, dividing. 

 N, N', macronucleus ; 

 n, n', the two dividing 

 micronuclei; o, o', 

 mouth. (After Hertwig.) 



Paramecium thus affords some idea of the complexities of 

 structure and function which a cell may exhibit when it forms 

 the whole animal organism. The Pro- 

 tozoa are the simplest, though by no 

 means simple, animals. But the great 

 structural differences between Sphaerella 

 and Paramecium, though to a certain 

 extent representative of plants on the 

 one hand and animals on the other, are 

 not essentially diagnostic, because, as we 

 have suggested before, in the last analysis 

 FIO. 12. Position as- ^ is a matter of metabolism. And it is 

 sumed by conjugating largely for this reason that Sphaerella and 



Paramecia. 



Paramecium, organisms shorn of all, or 

 nearly all, non-essentials, have been selected as illustrations. 



