20 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



the protoplasm around it is changing ; this is the 

 nucleus (n), and within it is a smaller body, the 

 little nucleus, or niicleolus. In the ectosarc we 

 have to observe a space which opens slowly, and con- 

 tracts rapidly ; its power of contraction may be seen 

 to be independent of that of the general mass of proto- 

 plasm. This space (the contractile vacuole, cv) 

 appears, though we cannot speak with certainty, to 

 be a kind of pump, whereby water is taken into and 

 forced out of the body ; the water that enters must 

 bring with it a certain quantity of oxygen, which is 

 a prime necessity of every living organism, whether 

 it be plant or animal ; while the water that is forced 

 out of the body must carry with it a certain quantity 

 of those waste products which always appear when a 

 living body is in active function. 



The contractile vacuole, then, would appear to 

 effect for the amoeba the two processes of respiration 

 and of purification, which, in higher animals, are per- 

 formed by definite organs. 



It will at once be noticed that there is no special 

 point by which food enters, or what is useless in that 

 food escapes from the amoeba ; in other words, there 

 is neither mouth nor anus. But it will almost as 

 soon be seen that this naked cell has no need of 

 either the one or the other ; it flows around the food 

 it needs, and it flows away from the waste or useless 

 matter which is of no further use to it. 



Just as there is no special inlet for the food, so 

 there is no part of the cell which can be said to be es- 

 pecially digestive in function. We can best see what 

 happens to the food when it is a green-coloured plant ; 

 when such is under observation we find that it 

 gradually breaks up within the amoeba, that it 

 gradually loses its green colour, and finally disappears ; 

 if it be a diatom that has been flowed around, we 

 may observe in time that the undigested case will be 



