33 MANUAL OF ZOOLOGY. 



cessive contractions and expansions of a muscular foot; but 

 some possess the power of swimming freely by means of a 

 modified fin-like foot. 



In most of the Gasteropoda the body is unsymmetrical, and is 

 coiled up spirally, " the respiratory organs of the left side being 

 usually atrophied " (Woodward). The body is enclosed in a 

 " mantle," which is not divided into two lobes as in the Lamel- 

 libranchiala, but is continuous round the body. Locomotion 

 is effected by means of the " foot," which is usually a broad 

 muscular disc, developed upon the ventral surface of the body, 

 and not exhibiting any distinct division into parts. In the 

 Hctcropoda, however, and in the Wing-shells (Strombickz), the 

 foot exhibits a division into three portions an anterior, the 

 " propodium ; " a middle, the " mesopodium ; " and a posterior 

 lobe, or " me'tapodium." 



In some, again, the upper and lateral surfaces of the foot 

 are expanded into muscular side-lobes, which are called " epi- 

 podia." In many cases the metapodium, or posterior portion 

 of the foot, secretes a calcareous, homy, or fibrous plate, 

 which is called the " operculum " (fig. 134, d), and which serves 

 to close the orifice of the shell when the animal is retracted 

 within it. 



The head in most of the Gasteropoda is very distinctly 

 marked out, and is provided with two tentacles and with two 

 eyes, which are often placed upon long stalks. Very often 

 there is an elongated retractile proboscis with ear-sacs, contain- 

 ing otoliths, at its base. The mouth is some- 

 times furnished with horny jaws, and is always 

 provided with a singular masticatory appara- 

 tus, called the "tongue" or " odontophore" 

 (fig. 132). " It consists essentially of a cartila- 

 ginous cushion, supporting, as on a pulley, an 

 elastic strap, which bears a long series of trans- 

 versely-disposed teeth. The ends of the strap 

 are connected with muscles attached to the 

 upper and lower surface of the hinder extremi- 

 ties of the cartilaginous cushions ; and these 

 muscles, by their alternate contractions, cause 

 the toothed strap to work backwards and for- 

 odonto- wards over the end of the pulley formed by its 

 k 6 (Buc- anterior end. The strap consequently acts, after 

 mag^fieT(After ^ f asmon f a chain-saw, upon any substance 

 Woodward.) to which it is applied, and the resulting wear 



and tear of its anterior teeth are made good by the incessant 

 development of new teeth in the secreting sac in which the 



