RHIZOPODA. eg 
be a rudimentary organ by which this fluid is driven through the | 
body. Ifwe watch an Ameéa (fig. 4, c) 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 probably 
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 vesi- 
cle of the Ameéa is to be looked upon as the first indication 
which we have in the entire animal kingdom of that most im- 
portant organ, the heart. 
The Ameoa possesses no breathing-organs of any kind, and 
no excretory organs, so that these functions must be performed 
by the general surface of the body in a manner somewhat the 
same as the exhalation from the skin which takes place in the 
higher animals. 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 nucleus. The nucleus (fig. 4, ) 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 means employed by the Ame@da 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” (Lat. jizdo, I cleave), and it consists in a gradual divi- 
sion 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 Amba, but this does not 
differ essentially from the former. 
Regarding the Ameéa 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 
c 
