130 GASTEROPOD CRAWLERS. 



When thus suspended they will sometimes relax their 

 hold and drop at once to the bottom, from which, in general, 

 they emerge by crawling up some solid body ; but occasion- 

 ally I have seen them rise up direct through the water a 

 fact I can explain only by supposing that they have the 

 power of compressing, in the first instance, the air in their 

 pulmonary cavity, and of again allowing it to expand and 

 dilate so as to render the body lighter than the medium in 

 which they live. * 



There are many exceptions in the class of Gasteropods, 

 as I have already told you, to our general description of the 

 foot, and these necessarily draw with them certain peculiari- 

 ties in the habits of the animals. In some genera (Pedipes, 

 Assimenia), the foot is divided by a transverse groove into 

 two unequal halves ; and in others, the place of the groove 

 is occupied by a muscular band distinguished by its greater 

 density and opacity. The latter structure is to be observed 

 in some common Rissose, which seem, however, to progress 

 in the usual fashion, but the structure affects the Pedipes 

 remarkably. When it desires to advance it makes fast by 

 the posterior half of the foot, and carries forward the an- 

 terior half as far as the groove (which relaxes considerably) 

 will permit ; then the creature draws forward the posterior 

 half so as to touch the anterior, and thus the body is ad- 

 vanced a space equal to that which separated these points. 

 This first step made, it begins a second, taking as the point 

 of support the posterior half, while the anterior is pushed 

 forwards, and when this is fixed in front, again dragging 

 towards it the posterior one. This movement, similar to 

 that of certain caterpillars, is executed with such quickness 

 that few mollusks excel the Pedipes in alertness, f The 

 locomotion of the Pupa pagodula is not very unlike this, 

 but the structure of the foot does not show the same pecu- 

 liarity. The snail is remarkably small in proportion to its 

 shell, the relation between them being made good by the 

 superior strength of the muscles of the foot, and of the 



action of the foot ; and he ascribes the motion to the action of the vibra- 

 tile cilia which cover the entire body, as well as the sole of the foot. Ann. 

 des Sc. Nat. (1843), xix. 309. I cannot assent unhesitatingly to this ex- 

 planation, for it seems to me, irreconcileable with some of the phenomena. 

 An Eolis crossing a basin can at once stop, and remain there for any time ; 

 but during all this period of rest, the cilia are in as active a state as when 

 the creature was in motion. 



* In the Ampullaria it is said that there is a large pouch in the roof of the 

 respiratory cavity, which is filled with air, and must serve as a kind of buoy 

 or swimming-bladder. 



t Adanson's Senegal, 13, and Introd. li. 



