SUBKINGDOM PUOTOZOA. 15 



By throwing out such a process and flowing after it, as it 

 were, locomotion is performed, which from a well-known 

 genus of the class is termed amoeboid. Food is simply en- 

 gulfed by the protoplasm flowing around it, after it has come 

 in contact with a pseudopodium, and the digestion of the 

 food-substance takes place within the protoplasm, being thus 

 iutracellular. Uudigestible material is discarded at any 

 part of the body ; respiration and excretion are carried on by 

 the general surface ; and reproduction is limited to the sim- 

 ple process of division. 



It is rare, however, to find such a simple condition as 

 this ; even among the simpler forms a certain differentiation 

 of the protoplasm exists, and it is doubtful if it is really 

 absent in any of the forms known to us. The structural dif- 

 ferentiations most usually occurring are the nucleus (Fig. 3, n) 

 and the contractile vacuole (Fig. 3, cv). The former, as was 

 noticed in the preceding chapter, is of great importance to 

 the cell, and it is questionable whether it is really absent 

 even in those Rhizopods in which it has not yet been dis- 

 covered. It is presumable, of course, that it is a structure 

 which has gradually become elaborated, that has evolved, 

 and that in the simplest conceivable organism it may have 

 been undifferentiated, but whether such an organism now 

 exists is questionable. The contractile vacuole is excretory 

 in its function, fluid containing products of metabolism in 

 solution accumulating at one or more definite regions of the 

 protoplasm to form it, and being by the sudden and rhythmi- 

 cal contraction of the surrounding protoplasm periodically 

 expelled from the body. 



Various degrees of complexity are, however, found among 

 the Rhizopods, the higher forms presenting a considerable 

 degree of differentiation both in structure and in the modes of 

 reproduction, and three orders based upon structural charac- 

 teristics may be distinguished. 



1. Order Foraminifera. 



The Foramiuifera contains the simpler members of the 

 class. In the genus Amoeba (Fig. 3) are organisms presenting 

 the simple characters above alluded to, being simple naked 



