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.  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 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 mud.  “Requiescat in pace.”