But we must take
a little glance at mollusks, including our clams, snails, and
cuttle-fishes; and at the articulates, including annelids and
culminating in insects. The molluscan and articulate lines, though
divergent, are of great importance to us as throwing a certain
amount of light on vertebrate development; and still more as showing
how a certain line of development may seem, and at first really be,
advantageous, and still lead to degeneration, or at best to but
partial success.
When we compare the forms which represent fairly well the direction
of development of these three lines, a snail or a clam with an
insect and a fish, we find clearly, I think, that the fundamental
anatomical difference lies in the skeleton; and that this resulted
from, and almost irrevocably fixed, certain habits of life.
We may picture to ourselves the primitive ancestor of mollusks as a
worm having the short and broad form of the turbellaria, but much
thicker or deeper vertically. A fuller description can be found in
the "Encyclopaedia Britannica," Art., Mollusca. It was hemi-ovoid in
form. It had apparently the perivisceral cavity and nephridia of the
schematic worm, and a circulatory system. In this latter respect it
stood higher than any form which we have yet studied. Its nervous
system also was rather more advanced.
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