84 Self and the Phenomenon of Life
b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity “9x6”organism. Ingestion and egestion take place in any part of the organism,
as there is no specialized area of the cell that can be identified as mouth
or anus. Other than the nucleus, the other prominent structures in the
cytoplasm are the contractile vacuoles that contain water. Water enters
the cytoplasm by osmosis because the osmotic pressure inside the cell
is higher than that in the surroundings. The amoeba expels the accumu-
lated water by periodic contraction of the contractile vacuoles.
Amoebas take up as food other microorganisms such as algae,
diatoms, bacteria and other protozoans. They can distinguish food from
inedible materials and use different tactics in approaching different
food. For such a simple organism, the way they achieve their goal is
quite remarkable. The following example shows how an amoeba engulfs
a Euglena cyst (Fig. 4.1). The cyst is spherical and easily rolls away as
soon as the approaching amoeba touches it. After pursuing in vain for a
while, the amoeba changes tactics by stopping the forward movement.
Instead, one pseudopodium is sent out from each side to surround the
cyst. At the same time the cytoplasm moves over the immobilized cyst to
cover it. In this manner the amoeba easily swallows its prey.^11
Fig. 4.1. An amoeba engulfing a euglena cyst. Numbers indicate sequence of events.
[See Note 11; permission Columbia Univ. Press.]