MOLLUSCA PROPER. 181 



Mollusks. In many, however, there is either no shell at all, or 

 one so small that it would not generally be recognized as such ; 

 and in a few the shell is composed of several pieces (" multi- 

 valve "). In none, however, is the shell composed of two 

 pieces or " bivalve." The great majority of the Gasteropoda 

 are further distinguished by the great development of the 

 foot, which constitutes a broad, flattened disk 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. 82, o), and which serves to close the shell when the 

 animal is withdrawn within it. 



s 



FIG. 82. Gasteropoda. Ampullaria canaUeulata, one of the Apple-shells : 

 o Operculum ; s Eespiratory siphon. 



The head in most of the Gasteropoda, unlike the Bivalves, 

 is very distinctly marked out, and carries two long feelers, and 

 two eyes, often placed upon stalks (Fig. 85). 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. 82), or " lingual ribbon." This con- 

 sists essentially of a number of siliceous teeth, of different 

 shapes in different species, supported upon a kind of strap which 

 can be made to work backward and forward over a cartila- 

 ginous cushion, thus acting like a chain-saw, In addition to 

 the odontophore there are sometimes horny jaws as well. The 

 mouth leads by a gullet into a distinct stomach, which some- 

 times is provided with calcareous plates for grinding down 

 the food, The intestine is long, and always terminates in a 

 distinct anal aperture. Distinct salivary glands are usually 

 present, and the liver is well developed. 



