And conversely, for an animal cell there can be no more ideal
existence than to contain a vegetable cell, constantly removing
its waste products, supplying it with oxygen and starch, and being
digestible after death. For our present knowledge of the power of
intracellular digestion possessed by the endoderm cells of the lower
invertebrates removes all difficulties both as to the mode of entrance
of the algae, and its fate when dead. In short, we have here the relation
of the animal and the vegetable world reduced to the simplest and
closest conceivable form.
It must be by this time sufficiently obvious that this remarkable
association of plant and animal is by no means to be termed a case of
parasitism. If so, the animals so infested would be weakened, whereas
their exceptional success in the struggle for existence is evident.
_Anthea cereus_, which contains most algae, probably far outnumbers all
the other species of sea-anemones put together, and the Radiolarians
which contain yellow cells are far more abundant than those which are
destitute of them. So, too, the young gonophores of Velella, which
bud off from the parent colony and start in life with a provision of
_Philozoon_ (far better than a yolk-sac) survive a fortnight or more in
a small bottle--far longer than the other small pelagic animals.
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