10 



POWER OF LOCOMOTIOX. 



of motion in all possible 

 ways, so that this member 

 may be readily protruded, 

 retracted, or inflected at 

 every point. In some bi- 

 valves, or two-shelled crea- 

 tures, the expansion of the 

 foot is effected by a curious mechanism : the interior 

 of the organ being formed of a spongy texture, capable 

 of receiving a considerable quantity of water, which 

 the animal has the power of injecting into it, and of 

 thus increasinsT its size. 



Under side of the foot of a species of 

 Nassa, -vnth part of the shell in outline. 



/ 6 



Mollusk with shell: Buccinum undatum. a a, shell; h b, foot; c, opercu- 

 Itim ; d, posterior syphon ; e, edge of the mantle ; / /, tentacula; ; g, pro- 

 boscis, with the mouth. 



