Thalamophores
The Thalamophores, the sister-group of one-celled animals which
largely compose our chalk and much of our limestone, are
developed on the same principle. The earlier forms seem to have
lived in a part of the ocean where silica was scarce, and they
absorbed and built their protective frames of lime. In the
simpler types the frame is not unlike a wide-necked bottle,
turned upside-down. In later forms it takes the shape of a
spirally coiled series of chambers, sometimes amounting to
several thousand. These wonderful little houses are not difficult
to understand. The original tiny animal covers itself with a coat
of lime. It feeds, grows, and bulges out of its chamber. The new
part of its flesh must have a fresh coat, and the process goes on
until scores, or hundreds, or even thousands, of these tiny
chambers make up the spiral shell of the morsel of living matter.
With this brief indication of the mechanical principles which
have directed the evolution of two of the most remarkable groups
of the one-celled animals we must be content, or the dimensions
of this volume will not enable us even to reach the higher and
more interesting types. We must advance at once to the larger
animals, whose bodies are composed of myriads of cells.
The social tendency which pervades the animal world, and the
evident use of that tendency, prepare us to understand that the
primitive microbes would naturally come in time to live in
clusters. Union means effectiveness in many ways, even when it
does not mean strength. We have still many loose associations of
one-celled animals in nature, illustrating the approach to a
community life. Numbers of the Protozoa are social; they live
either in a common jelly-like matrix, or on a common stalk. 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. The deposit goes on
until at last an elaborate framework of thorny, or limy, or
flinty material is constructed by the one-celled citizens. In the
higher types a system of pores or canals lets the food-bearing
water pass through, as the animals draw it in with their lashes;
in the highest types the animals come still closer together,
lining the walls of little chambers in the interior.
|
