the simplest animals, consisting of a single cell, the protoplasm simply flows round the particle of food, and it is ‘ingested’ with a drop of water. Into this ‘food vacuole’ the ferments are secreted, and when all that is useful has been dissolved out and absorbed, the bubble moves to the surface and bursts; or, to put it differently,
Diagram 1.—The AmŒba.
the cell flows on its way, and the vacuole, with any shell or refuse it may contain, gets left behind. (See Diagram 1.) In other cells which are constant in shape there is an opening leading to the interior of the cell. Round this there are little projecting threads, which beat the water
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Diagram 2.—The ParamŒium.
regularly. In some positions these threads enable the cell to swim, but here their duty is to cause a current and wash particles of food down the primitive throat into the interior, where, as in the preceding case, they become enclosed in a vacuole. (See Diagram 2.)
Moving a stage higher, we find animals consisting of
