406 



MANUAL OF ZOOLOGY. 



and not exhibiting any distinct division into parts. In the 

 Heteropoda, however, and in the Wing-shells (StromMdce), the 

 foot exhibits a division into three portions : an anterior, the 

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

 lobe, or " metapodium." In the Heteropoda, the foot is flatten- 

 ed, and forms a ventral fin, by means of which the animal 

 swims, back downwards. 



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 



Fig. 212. A, Sketch of a Whelk (Buccinnm undatuin) in motion : f Foot ; h Head 

 carrying the feelers (f) with the eyes (e) at their bases ; / Proboscis ; s Respiratory 

 siphon, or tube by which water is admitted to the gills ; o Operculum. B, Shell of 

 the Whelk : a Spire ; b Body-whorl ; n Notch in the front margin of the mouth of 

 the shell ; nt Outer lip of the mouth of the shell. This figure is half the natural 

 size. C, A small cluster of the egg-capsules of the Whelk. (B and C are after 

 Woodward.) 



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

 which is called the "operculum " (fig. 215, <?), 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 



