232 AMOEBA CHAP. 



frog is, as we have seen, a multicellular animal, built up 

 of innumerable cells which are incapable of an indepen- 

 dent existence for any length of time. 



Besides the nucleus, there is another structure fre- 

 quently visible in the living Amoeba and not present in 

 the leucocyte. This is a clear, rounded space in the 

 protoplasm (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 containing a watery fluid. 



We must now study the physiology of our animal 

 cule. 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, 172), 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 animalcule 

 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 nitrogenous substance allied 

 in composition to horn and to the chitin. (see Chapters 

 VI VIII) which forms the external shell of crustaceans, 

 insects, &c. 



