A TEXTBOOK OF PHYSIOLOGY 



structure, the nucleus. Observation shows that the amoeba gradually 

 moves about in the fluid here thrusting out one little foot-like mass, 

 or pseudopodium, there retracting another, and thus progressing. 

 If it be touched in any way, the pseudopodia are withdrawn. The 

 organism thus responds to a stimulus, and possesses what is termed 

 irritability. Should there, perchance, be any food material present, 

 such as algse, it may be seen how, by means of the pseudopodia, the 

 food is surrounded and gradually absorbed into the protoplasmic 

 mass. This food is gradually broken down, digested, and assimilated, 

 any indigestible or waste material being later extruded from the 

 surface. From time to time the vacuole contracts, voiding by this 

 process waste products of the animal's activity. Finally, under 

 favourable circumstances, the little animal may be seen to grow, and 

 eventually divide into two organisms, thereby reproducing itself (Fig. 2). 



C 



! * / 



FIG. 2. SUCCESSIVE CHANGES EXHIBITED BY AN AMCEBA. 



Anatomy.") 



(Venvorn from "Quain's 



The amoeba is a type of what is known as a unicellular organism; 

 it consists of but one unit or cell. That one cell performs all the 

 life-processes. All organisms, however, are not so simple in structure ; 

 the higher forms of life consist of a great number of cells, and are 

 therefore termed muiticellvlar. The multicellular organisms have 

 been evolved from the simpler unicellular one in some cases through 

 an almost infinite number of stages. Past history, or phylogeny, 

 as shown by fossil remains, indicates that, as the ages passed 7 

 animals gradually became more and more complex in structure. 

 Some of the simpler forms have continued to exist, others have become 

 lost. So, too, of the evolved multicellular forms many continue 

 with us, but some have passed away. The ability of an organism to 

 maintain its life depends upon its power to adapt itself to existing 

 conditions upon its being efficient. The multicellular organism 

 developed its efficiency over the unicellular organism along two lines: 



