[Illustration: /Sigaretus lœvigatus/, showing excessive development of the propodium (/pr./) and metapodium (/met./) in a mollusk living in sand (the shell, which covers only the liver and adjacent parts, has been removed): /l/, liver; /s. ap./, aperture of proboscis, here deflected from the median line; /t/, /t/, tentacles; /f/, foot.] [Illustration: /Oliva textilina/ showing how the front part of the foot (/f/) is developed into a sort of fender, the propodium (/pr./): /e/, /e/, eyes; /m. ap./, front appendage of mantle; /m. ap.´/, hinder appendage of mantle, folded into the suture when the animal is at rest; /si./, siphon; /t/, /t/, tentacles.] There is no single feature of mollusks more important than the foot, for upon the many modifications of this organ the various molluscan classes are founded. The name /Gasteropoda/ means "stomach-foot," the latter organ being merely a thickening of [pg336] the ventral surface of the animal, giving to the creature, when active, the appearance of crawling upon its stomach. If a living gasteropod is placed in a jar of sea-water and left undisturbed it will soon crawl up the side of the glass and thus afford an excellent view of the extended under surface of its foot. The majority of the gasteropods are lethargic and slow in movement, but others will be found to be exceedingly lively and able to creep up the sides of the jar and make their escape in a surprisingly short time. Further modifications of the foot will be mentioned later. THE MANTLE CAVITY Now let us place the animal with the foot below and the head facing us. The spiral visceral hump will point back and away from the observer. The mantle covering the visceral portion is thin and translucent like an ordinary skin, but at the base of the visceral mass the mantle suddenly thickens very greatly, and spreads over the dorsal portion of the foot or the body like a cloak. The space between the body and the loose-lying mantle is called the /mantle cavity/, and within this area are to be found several important external organs. To find these it is well to cut the mantle flap in a straight line, beginning just back of the head and ending at the point where the mantle is attached to the body. This will most easily be done with scissors. Having thus divided the mantle in front, throw back the two flaps. On the right-hand side (left of the animal), and attached to the inner side of the mantle thus exposed, will be seen the branchiæ, or gills. THE BRANCHIÆ, OR GILLS