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.
Here we have a very clear evolutionary transition from the
solitary microbe to a higher level, but, unfortunately, it does
not take us far. The Sponges are a side-issue, or cul de sac,
from the Protozoic world, and do not lead on to the higher. Each
one-celled unit remains an animal; it is a colony of
unicellulars, not a many-celled body. We may admire it as an
instructive approach toward the formation of a many-celled body,
but we must look elsewhere for the true upward advance.
The next stage is best illustrated in certain spherical colonies
of cells like the tiny green Volvox (now generally regarded as
vegetal) of our ponds, or Magosphoera. Here the constituent cells
merge their individuality in the common action. We have the first
definite many-celled body.
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