Such
instances, which might easily be multiplied, show that the association
is beneficial to the animals concerned.
The nearest analogue to this remarkable partnership is to be found in
the vegetable kingdom, where, as the researches of Schwendener, Bornet,
and Stahl have shown, we have certain algae and fungi associating
themselves into the colonies we are accustomed to call lichens, so that
we may not unfairly call our agricultural Radiolarians and anemones
_animal lichens_. And if there be any parasitism in the matter, it is
by no means of the alga upon the animal, but of the animal, like the
fungus, upon the alga. Such an association is far more complex than
that of the fungus and alga in the lichen, and indeed stands unique in
physiology as the highest development, not of parasitism, but of the
reciprocity between the animal and vegetable kingdoms. Thus, then, the
list of supposed chlorophyl containing animals with which we started,
breaks up into three categories; first those which do not contain
chlorophyl at all, but green pigments of unknown function (_Bonelia<,
Idotea_, etc.); secondly, those vegetating by their own intrinsic
chlorophyl (_Convoluta_, _Hydra_, _Spongilia_); thirdly, those
vegetating by proxy, if one may so speak, rearing copious algae in their
own tissues, and profiting in every way by the vital activities of
these.
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