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
than almost any worm. It has a supra-oesophageal ganglion of fair
size.

The clams and oysters show even more clearly what we might call the
logical results of molluscan structure. They increased the shell
until it formed two heavy "valves" hanging down on each side of the
body and completely enclosing it. They became almost sessile, living
generally buried in the mud and gaining their food, consisting
mostly of minute particles of organic matter, by means of currents
created by cilia covering the large curtain-like gills. Their
muscular system disappeared except in the ploughshare-shaped "foot"
used mostly for burrowing, and in the muscles for closing the shell.
That portion of the body which corresponds to the head of the snail
practically aborted with nearly all the sense-organs. The nervous
system degenerated and became reduced to a rudiment. They had given
up locomotion, had withdrawn, so to speak, from the world; all the
sense they needed was just enough to distinguish the particles of
food as they swept past the mouth in the current of water. They have
an abundance of food, and "wax fat." The clam is so completely
protected by his shell and the mud that he has little to fear from
enemies. They have increased and multiplied and filled the m