thicker or deeper vertically. A fuller description can be found in
the "Encyclopædia 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. It had apparently already
taken to a creeping mode of life and the muscles of its ventral
surface were strongly developed, while its exposed and far less
muscular dorsal surface was protected by a cap-like shell covering
the most important internal organs. But the integument of the whole
dorsal surface was, as is not uncommon in invertebrates, hardening
by the deposition of carbonate of lime in the integument. And this
in time increased to such an extent as to replace the primitive,
probably horny, shell.

Into the anatomy of this animal or of its descendants we have no
time to enter, for here we must be very brief. We have already
noticed that the most important viscera were lodged safely under the
shell. And as these increased in size or were crowded upward by the
muscles of the creeping disk, their portion of the body grew upward
in the form of a "visceral hump." Apparently the animal could not
increase much in length and retain the advantage of the protection
of the shell; and the shell was the dominating structure. It had
entered upon a defensive campaign. Motion, slow at the outset,
became more difficult, and the protection of the shell therefore all
the more necessary. The shell increased in size and weight and
motion became almost impossible. The snail represents the average
result of the experiment. It can crawl, but that is about all; it is
neither swift nor energetic. Even the earthworm can outcrawl it. It
has feelers and eyes, and is thus better provided with sense-organs