class, according to Dr. Verworn, make up a considerable
part of the life of Protista.

"A Stentor draws itself suddenly together, stretches itself out,
draws in again after a short time, may remain stretched out a
long time without the slightest contraction, till presently it gives
vent to a whole series of contractions at irregular intervals."

There is no appearance of purpose in these irregular
movements, and Dr. Verworn ¹ points out that by im-
perceptible degrees they pass over into periodical, that is to
say, automatic movements. Automatic movement indeed,
as we see it in the pulsating vacuole among Protista, and
among higher animals in the beating of the heart or the
action of the respiratory centre, is a development and
adaptation of the persistent structural activity to special
requirements of the organism. It is due to the inter-
action of internal forces (though it may be modified by
outer influences) but it is repeated at regular intervals, or
rather, like the beating of the heart, it forms a connected
cycle of events each of which introduces the next, till at
length the cycle is completed and starts afresh.

To understand movements of this class we must bear
in mind that in any living cell chemical changes are
constantly going on. The protoplasm of the cell is
building itself up out of materials supplied by the
surrounding medium. This is the assimilative process.
At the same time an opposite process of dissimilation
is going on, in which the constituents of the protoplasm
form new combinations, and ultimately become waste
products of which the cell gets rid. These changes
involving a constant molecular movement, the cell is
never in a condition of stable equilibrium. But the outer
results may be very different, according as the internal
changes do or do not tend to balance one another.
If they merely oscillate slightly about the point of
equilibrium, there will be no outward sign of change.
If there is a slow but decided gain of one set of forces
in one part of the cell, an amœboid movement will
result. The cell will slowly put out a projection or

-39-