The ideal mollusk has a compact, somewhat elongated body, the under or ventral portion of which is thickened into a muscular disk, upon which it creeps along. This disk is called the /foot/. (Note a snail thus crawling upon his "foot.") The foot is rounded or obtusely pointed posteriorly. Forward, the body is slightly narrowed into a neck, and then expanded again into a head. Upon this head are carried two projecting, mobile, fleshy tentacles, one upon each side, that superficially resemble two horns. At the base of each tentacle, upon the inner side, is an eye. Just below and between the tentacles is a small horizontal slit,—the mouth,—the lips being slightly thickened. The visceral portion of the animal is above the foot, or /dorsal/ to it, and extends the whole length of the animal from the mouth to the excretory opening, the latter being situated posteriorly. The visceral cavity is inclosed above by a thin skin; indeed, it is but the great thickening of this skin below that constitutes the foot. [Illustration: Ideal or schematic mollusk. 1, dorsal view: /h/, head; /t/, cephalic tentacle; /sh./, shell covering visceral mass; /b.m./, outline of body-mass; /r.m.e./, edge of mantle; /c.t./, ctenidia, or gills; /q.a./, generative apertures; /r.e./, renal openings; /e.o./, excretory opening; /f/, posterior portion of foot; /o.e.m./, edge of shell. 2, ventral view: /h/, head; /t/, tentacle; /m/, mouth; /b/, outline of body; /e.m./, edge of mantle; /s.f./, lower surface of foot; /c.t./, ctenidia, or gills; /f/, posterior portion of foot. 3, horizontal section, showing heart, nephridia, and gonads: /m/, mouth; /o/, esophagus; /g/, gonad; /p/, pericardium; /n/, nephridium, or renal organ; /v/, ventricle; /a/, auricles; /g´/, gonad aperture; /n´/, renal opening; /e/, excretory opening; /f/, posterior portion of foot. 4, vertical median section: /m/, mouth; /b.m./, buccal mass; /l/, liver; /d.a./, digestive tract; /q/, gonad; /f/, foot; /q.o./, gonad aperture, or opening of oviduct; /r.o./, renal opening; /e.o./, end of digestive tract, excretory opening; /n/, mantle; /r/, nephridium, or renal organ; /o.f./, recurved portion of mantle; /sh./, shell; /v/, ventricle; /p/, pericardium.]