liHIZOPODA. 27 



t}ie body. If we watcli an Amoeba (fig. 6, B) 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 regular 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 

 " contractile 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 pi'obably with the nutritive 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 Amceba is to be looked upon as 

 the first indication which we have in the entire animal kingdom of 

 that most important organ, the heart. 



The Amosha possesses no breathing - organs of any kind, and \w 

 excretory organs, so that these functions must be performed by thu 

 general surface of the body in a maimer somewhat the same as the 

 exhalation from the skin which takes place in the higher animal;'. 

 There are, also, no traces of a nervous system, and no organs of 

 sense, and the only other structure of any kind is what is known 

 as the inicleus. The nucleus (fig. 3 and fig. 6 n) is simply a small 

 rounded or oval, granular mass, and there may be more than one 

 in the same individual. Its function, however, is quite unknown, 

 though it is probably connected with reproduction. The meaiia 

 employed by the Amoeba to perpetuate the species are various, but 

 the only one which need be mentioned is the process by self-division. 

 This is what is technically called " fission " (hixt. findo, I cleave), and it 

 consists in a gradual division or cleavage of the body into two parts, 

 each of which then becomes a separate and independent individual. 

 In some cases this process is slightly varied, a single pseudopodium 

 alone being cast off and becoming a fresh Amoeba, but this does not 

 differ essentially from the former. 



Begarding the Amceba from a physiological point of view, we see 

 that, though the animal nourishes itself and maintains its existence 

 perfectly, the process of nutrition is carried on in the simplest pos- 

 sible manner, and with the simplest possible apparatus. There is 

 no pemiatient mouth, no stomach or alimentary canal of any kind, 

 no respiratory or excretory organs, and even no distinct aperture 

 for the extrusion of indigestible food. The only distinct structure 

 which is at all concerned in imtrition is a rudimentary contractile 

 cavity, the first foreshadowing of the heart in the higher animals. 



