232 AMCEBA CHAP. 



a multicellular animal, built up of innumerable cells which are 

 incapable of an independent existence for any length of time. 



Besides the nucleus, there is another structure frequently 

 visible in the living Amoeba and not present in the leuco- 

 cyte. This is a clear, rounded space in the ectoplasm (c. vac), 

 which periodically disappears with a sudden contraction and 

 then slowly reappears, its movements reminding one of the 

 beating of a minute colourless heart. It is called the 

 contractile vacuole, and consists of a cavity in the ectoplasm 

 containing a watery fluid. 



We must now study the Physiology of our animalcule. 

 First of all, as we have already seen, it is contractile : 

 although it has no muscles, it can move about from place to 

 place. Its movements, like the voluntary movements of the 

 frog (pp. 7, 173), may occur without the application of any 

 external stimulus, i.e., they are spontaneous or automatic; or 

 they may be induced by external stimuli — by a sudden 

 shock or by coming in contact with an object suitable for 

 food. Movements of this latter kind, like those resulting 

 from the stimulation of the nerves in a brainless frog, are 

 the result of the irritability of the protoplasm ; the animal- 

 cule is therefore both automatic and irritable, although it 

 possesses neither nerves nor sense-organs. 



Under certain circumstances an Amoeba temporarily loses 

 its power of movement, draws in its pseudopods, and 

 becomes a globular mass around, which is formed a thick, 

 shell-like coat, called the cyst or cell-wall (Fig. 67, d, cy\ 

 This is formed by the protoplasm by a process of secretion 

 (p. 130) : its composition is not known ; it is certainly not 

 protoplasmic, and very probably consists of some nitro- 

 genous substance allied in composition to horn and to the 

 chitin (see Chapters V & VI) which forms the external shell 

 of crustaceans, insects, &c. 



