PROTOZOA TO WORMS 



33 



tracts. It recognizes its food even at a microscopic 



distance; it appears therefore to feel and perceive. 



Perhaps we might say that it has a mind and will of 



its own. It is safer to say that it is irritable, that is, 



it reacts to stimuli too feeble to be regarded as the 



cause of its reaction. 



It engulfs microscopic 



plants, and digests them 



in the internal proto- 



plasm by the aid of 



an acid secretion. It 



breathes oxygen, and 



excretes carbonic acid 



and urea, through its 



whole body surface. Its 



mode of gaining the en- 



ergy which it manifests 



is therefore apparently 



like our own, by com- 



bustion of food mate- 



rial. 



It grOWS and reaches 



L 



FHOTBUB. HERTWIG, PROM LEIDT. 



j j i ek, ectosarc ; en, endosarc ; iV, food par- 



a certain size, then con- tides . n> n ' ucleus . cv> contractile vesic ' e . 

 stricts itself in the mid- 



dle and divides into two. The old amoeba has divided 

 into two young ones, and there is no parent left to die, 

 and death, except by violence, does not occur. But 

 this absence of death in other rather distant relatives 

 of the amoeba, and probably in the amoeba itself, holds 

 true only provided that, after a series of self-divisions, 

 reproduction takes place after another mode. Two 

 rather small and weak individuals fuse together in one 

 animal of renewed vigor, which soon divides into two 

 3 



