14 BRITISH FRESHWATER RHIZOPODA. 
and the expulsion of the contents, step by step, under 
a high power. Its reappearance he found to be always 
somewhere near its point of disappearance. ‘ While 
still small it is carried along by the streaming proto- 
plasm back to a position near the nucleus, where it 
completes its development. The increasing weight of 
the growing vacuole causes it to lag behind the stream- 
ing granules and nucleus, until at its full growth it is 
widely separated from the latter organ. The vacuole 
may appear to move in the direction contrary to that 
of the protoplasmic streaming, although in reality it is 
quiescent; for while it remains in the field of the 
microscope the main body of the animal moves well 
out of it, until the vacuole is surrounded only by the 
posterior end of the animal, which is reduced to a thin 
layer of granules and a hyaline layer of ectoplasm 
between the vacuole and the exterior. The granules 
later move away, passing around the vacuole, until finally 
there is only a thin layer of hyaline plasm between the 
vesicle and the exterior. Shortly after this the vacuole 
bursts and disappears, in most cases a distinct bulge 
toward the outside preceding contraction. Contrac- 
tion always begins on one side of the vacuole, and is 
carried across it toward the outer edge.”* 
The action of the contractile vesicle, doubtless, has 
an important physiological meaning. The generally- 
accepted view is that the periodical discharge is an 
excretory function. The excess of water in the plasma- 
body drains mto the vacuole, and is thus got rid of. 
In the more highly-organised Infusoria the water is 
conveyed to the excretory vacuole by a system of ducts. 
Such channels have not been seen in the endoplasm of 
the Rhizopoda; but whatever the means employed by 
Nature in their case may be, the effect is the same. 
Inception or Foon. 
The food of the Rhizopoda is, in the main, chloro- 
phyllous. It consists of diatoms, desmids, and spores 
* Calkins, op. cit., p. 88. 
