160 UNICELLULAR ANIMALS. 



sac, but the two substances are not separated by any definite 

 boundary-line, and pass imperceptibly into one another. The 

 external boundary of the body is formed by the outermost limit 

 of the ectoplasm. There is no membrane, and the body is 

 quite naked. Nevertheless the protoplasmic mass shows no 

 tendency to mix with the surrounding water, and perfectly main- 

 tains its integrity ; it is an individual. 



The formation of a pseudopod begins by the bulging out of 

 the ectoplasm to form a rounded prominence at some point on 

 the surface. Into its interior a sudden gush of entoplasm then 

 takes place and a steady outward stream ensues, the entoplasm 

 pushing the ectoplasm before it, and the substance of the body 

 flowing into the pseudopod. The whole substance of the body 

 may thus flow onward into the pseudopod, which meanwhile forms 

 new pseudopods, and so the entire animal advances in the direction 

 of the flow ; or, the pseudopod after attaining a certain size may 

 be withdrawn into the body by reverse (centripetal) currents, the 

 main body having meanwhile flowed onward in another direction. 



As a rule, the new pseudopodia are put forth near one end 

 of the body (hence called " anterior "), and the general direction 

 of advance is therefore fairly constant, not vague and indefinite, 

 as is often stated. The direction of flow fluctuates, however, 

 about a certain mean, being continually diverted this way or 

 that by the formation of new pseudopodia. Those which do not 

 form directly in the line of march either merge little by little 

 with the advancing ones, or are withdrawn bv reversed currents 



O v 



into the body. In the latter case they often leave shrivelled 

 wart-like remnants, and a group of similar warts is usually 

 found near the "posterior' end of the body (Fig. 84, p). 

 Definite changes in the general direction of advance are eifected 

 by the diversion of the main current into lateral pseudopodia. 



Amoeba feeds upon minute plants and animals or other or- 

 ganic particles. There is no mouth, and food-matters are bodily 

 ingulfed (at no definite point) by the protoplasm which closes 

 up beyond them.* The indigestible remains are passed out in 



* This mode of cellular alimentation is of frequent occurrence in some cells 

 of multicellular, as well as in unicellular, animals. Cells exhibiting it are 

 known as phagocytes (eating-cells), and the process is referred to as phagocytosis. 

 It is obviously only a prelude to intra-cellular digestion. 



