In
fact, we have a singularly instructive illustration of the
process in the evolution of the sponges.
It is well known that the horny texture to which we commonly give
the name of sponge is the former tenement and shelter of a colony
of one-celled animals, which are the real Sponges. In other
groups the structure is of lime; in others, again, of flinty
material. Now, the Sponges, as we have them to-day, are so
varied, and start from so low a level, that no other group of
animals "illustrates so strikingly the theory of evolution," as
Professor Minchin says. We begin with colonies in which the
individuals are (as in Proterospongia) irregularly distributed in
their jelly-like common bed, each animal lashing the water, as
stalked Flagellates do, and bringing the food to it. Such a
colony would be admirable food for an early carnivore, and we
soon find the protective principle making it less pleasant for
the devourer. The first stage may be--at least there are such
Sponges even now--that the common bed is strewn or sown with the
cast shells of Radiolaria. However that may be, the Sponges soon
begin to absorb the silica or lime of the sea-water, and deposit
it in needles or fragments in their bed.
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