198 INVERTEBRATE ANIMALS. 
CLASS IJ. GASTEROPODA.—This class includes an enormous 
number of Molluscs, such as the land-snails, sea-snails, whelks, 
limpets, slugs, sea-lemons, &c., which agree in many funda- 
mental characters, but nevertheless present many striking dif- 
ferences. From the very common occurrence of a shell com- 
posed of a single piece, the Gasferopoda are often spoken of in 
a general way as the “univalve” Molluscs. In many, however, 
there is either no shell at all, or one so small that it would not 
generally be recognised as such ; and in a few the shell is com- 
posed of several pieces (‘multivalve”). In none, however, is 
the shell composed of two pieces or “bivalve.” The great 
majority of the Gasterofoda are further distinguished by the 
great development of the foot, which constitutes a broad flat- 
tened disc upon which they creep about, as may readily be 
observed in the common slugs. Some,-however, have the foot 
much modified and adapted for swimming. In many cases, 
also, the foot carries behind a horny or shelly plate which is 
known as the “operculum” (fig. 102, 0), and which serves to close 
the shell when the animal is withdrawn within it. 
Fig. 1o2.—Gasteropoda. <Asmpullaria canaliculata, one of the Apple-shells : 
o Operculum ; s Respiratory siphon. 
The head in most of the Gasterofoda, unlike the Bivalves, is 
very distinctly marked out, and carries two long feelers, and 
two eyes, often placed upon stalks (fig. 102). The mouth, also, 
differs from that of the Bivalves in being furnished with a 
singular apparatus of teeth, constituting what is known as the 
“ odontophore” (fig. 103), or “lingual ribbon.” This consists 
essentially of a number of silicious teeth, of different shapes in 
