GASTEROPOD MOLLUSCA. 127 



incapable of being applied in a horizontal position to a flat 

 surface. 



The Gasteropods crawl along at a pace proverbially slow, 

 — *' slow crawls the snail," — • but there are degrees of slug- 

 gishness among them, and it may be regarded as generally 

 holding true, that the narrower and more elongated the foot, 

 the quicker the motion, which becomes retarded just as the 

 organ tends more to the oval or round. Thus the snail 

 greatly excels in his paces the shield-like limpet, and the 

 slug beats the Doris. A foot either disproportionably large 

 or small is also a hinderance to progression. The Cypraeae 

 and the Olivae have a foot considerably larger than the shell, 

 with a deep furrow run down its centre, and these are slug- 

 gish genera. The Yet of Adanson (Cymba neptuni) has 

 what we would call a monstrous foot, being as broad again 

 as the shell and one half longer when expanded ; nor can it 

 be withdrawn within the shell by any effort of the animal. 

 We may conclude it to be tardigrade. In the Cones, on the 

 contrary, the foot is small and of weak muscular power, — yet 

 so rich is all creation with proofs of contrivances adapted to 

 annul a defect, that we might anticipate to find some remedy 

 here, — and it is so. The mouth of the snail is situated in a 

 cavity, and this cavity it apjilies to aid its weakness ; for it 

 performs, Hke the oral aperture of the leech, the office of a 

 sucker, by which the head is readily affixed to foreign bo- 

 dies. Thus the animal facilitates its progress, and is enabled 

 to drag along the shell, of a weight and size otherwise quite 

 burdensome to it.* 



The foot of Gasteropods is mostly of a uniform structure 

 throughout, — that is to say, its muscular fibres are interwoven 

 much in the same manner as they are in the human tongue, 

 and are not collected into distinct and separate bands. There 

 are, however, several considerable exceptions to this. In 

 the slugs and snails progression is performed by a pair of 

 muscles which extend from the tail to the fore part, running 

 along the middle of the foot, while the sides are formed of 

 strong transverse fibres ; in the Pedipes and its allies there 

 is a transverse mviscular band under the neck which sepa- 

 rates the foot into an anterior and posterior portion ; and in 

 some land-snails (Cyclostoma) the foot is divided longitudi- 

 nally into two equal halves. By a series of undulations pro- 

 pagated along the sole, or these muscular bands, from the tail 

 forwards to the head, occupying sometimes the whole sur- 

 face, and sometimes the middle only, and resembling, to use 

 the apt comparison of Swammerdam, " the waves and bil- 

 * Adanson, Senegal, 89. 



