MOLLUSCA — GASTROPODS 243 



drawn out into a canal (as Murex, etc.). Most of the former 

 likewise have many and small teeth upon the radula, while in 

 the latter they are few but strong. 



Experiment has shown that to aquatic gastropods the form of 

 objects is indistinguishable. Terrestrial species can distinguish 

 form when distant only one to two millimeters. It has beer 

 shown that Polygyra albolabris, the common land snail, for 

 example, has so little perception of forms that it often creeps 

 directly against an object, changing its course only when the 

 tentacles touch it. The same animal is entirely unaffected by 

 any noise which does not actually jar its body. 



Sometimes the inner parts of the whorls coalesce into a colu- 

 mella. When they do not thus unite but leave a central tubular 

 cavity, the opening of this cavity below is called the umbilicus. 

 In some cases the inner edge of the mantle is reflected back over 

 the inner edge of the aperture. This secretes a shelly growth 

 beneath it which partially covers the umbilicus and is called the 

 callus. 



In many forms the mantle protruding from the shell aperture 

 is reflected back over the shell to such an extent that it covers 

 more or less of its external surface as in Cypraea, and this re- 

 flected portion may secrete a coating of enamel over the part of 

 the shell on which it lies. The shell in these forms becomes 

 thus more or less inclosed within the soft body portion, a process 

 which attains such an extent in certain forms representing off- 

 shoots from various groups {e.g. Marseniidae from the Strepto- 

 neura, the slugs from the Pulmonata, the sea-hares from the 

 Opisthobranchia, etc.) that the animal appears to be quite naked. 

 In these the mantle not only covers the shell but its edges are 

 fused together over the top. The shell accordingly, having 

 almost ceased to be of use as a protection, degenerates into a 

 small plate, appearing entirely unlike the ordinary gastropod 

 shell. Finally the shell disappears entirely in the nudibranchs, 

 some of the pteropods and some of the air-breathing gastropods. 



As the shell is the only part preserved in fossil forms our knowl- 

 edge of the characteristics of extinct species and their relation- 



