RHIZOPODA. 31 



curely in the substance of the body. It follows from this that 

 the Amoeba has no permanent mouth no aperture, that is, 

 which is especially employed in the admission of food. Any 

 part of the surface can be pushed out into a pseudopodium, 

 and therefore any part of the surface can be extemporized 

 into a mouth. The process of taking food, in fact, in the 

 Amoeba, has been aptly compared to thrusting a stone or any 

 other solid body into a lump of dough. The central portion 

 of the body of the animal is softer and more fluid than the 

 outer layers, and the particles of food, on reaching this point, 

 undergo a sort of digestion, and are subjected to a species of 

 movement or rotation in the interior of the animal. Each 

 particle of food, in the process of being taken into the body, 

 usually carries with it a little drop of water ; and in this way 

 a number of clear spaces are formed, which are usually quite 

 round, and look like distinct cavities. These spaces are called 

 " vacuoles ; " but they are not distinct organs of any kind, 

 though formerly regarded as distinct stomachs. Having un- 

 dergone digestion, any portions of food which may be indigest- 

 ible or insoluble are simply thrust out again through the walls 

 of the body. This appears to be effected at one particular 

 part of the body ; but there is no permanent aperture for the 

 purpose. There are no distinct vessels which serve to convey 

 the nutritive fluid derived from the digestion ; but there does 

 appear to be a rudimentary organ by which this fluid is driven 

 through the body. If we watch an Amoeba carefully, there is 

 usually no difficulty in observing that every now and again 

 there appears at one particular place a clear spot, " like a 

 window," which slowly expands to its full extent, and then 

 usually contracts slowly till it disappears altogether. This 

 process of gradual expansion and contraction is what is called 

 " rhythmical " that is to say, it is repeated at tolerably regu- 

 lar intervals, perhaps twice a minute. In some cases the 

 vesicle, when contracted, remains so for a long time, but it 

 always reappears in the same place. It is known as the con- 

 tractile vesicle ; and there can be little doubt that it is a 

 permanent organ. It is, in fact, a little clear space or cavity 

 in the substance of the body, filled probably with the nutri- 

 tive fluid derived from the digestion, and no doubt serving by 

 its contraction to drive this fluid to various parts of the body. 

 In its function, then, the contractile vesicle of the Amoeba is 

 to be looked upon as the first indication which we have in the 

 entire animal kingdom of that most important organ, the 

 heart. 



