MALACOLOGY. 



83 



Some terrestrial molluscs progress by lifting 1 the an- 

 terior part of their body, and bringing up in tolerably 

 quick time the posterior part or foot, at the same 

 moment making a well marked step forward. A very 

 large proportion of molluscs crawl on the surface of 

 the earth, or in the water, by the aid of a foot or 

 ' muscular disc, with which their belly is furnished ; 

 >ut this kind of reptation differs widely from that of 

 reptiles ; it is rather a species of gliding forward, 

 produced by the rapid succession of the extremely 

 minute undulations composed of muscular fibres, 

 forming that part called the foot. 



It results from this species of motion, by which 

 every inequality of the ground or other substance is 

 touched as the animal passes over it, that the advance 

 is by no means rapid, and under any circumstance of 

 excitation cannot be materially increased ; though 

 the slugs and others of their class, whose foot is 

 very large, thick, and wide, get forward with greater 

 despatch than might at first be imagined ; and the 

 absence of a shell to carry must doubtless render their 

 motions more easy. Some, on the contrary, whose 

 foot is always very large, as the Patella and Haliotis, 

 move so slowly, and so seldom change their resting- 

 place, that many naturalists have erroneously sup- 

 posed them constantly stationary ; respecting these, 

 we have under their names pointed cut another fact, 

 that of the power they possess of adhering with im- 

 mense force to the object upon which they rest. The 

 Hypponyx and Capula remain fixed to one spot 

 during life ; and the foot, not being actively employed 

 in them, is scarcely to be called muscular ; it more 

 nearly resembles the horse-shoe-formed muscle which 

 attaches the back of the animal to its shell. The 

 Sci/lla;a, whose foot is extremely narrow, and as if 

 channelled, cannot without great difficulty move on 

 a plane surface ; but can with perfect facility and 

 even speed, climb the stems and peduncles of marine 

 plants ; its progress is, however, completely sliding 

 along them ; this is, doubtless, assisted by the 

 medium of water rendering the objects they are 

 found upon smooth and easily passed over ; to com- 

 pensate for which the slugs are abundantly supplied 

 with a lubricating fluid secretion, which enables them to 

 slip over the minute dry substances that would with- 

 out that wise provision encumber or clog their foot. 

 The common grey slug, here figured, will readily 

 explain this kind of locomotion. 



Limax cinereus. 



Those species of molluscs, which can ascend in water, 

 do so by making the water a point of resistance ; but 

 they are then compelled to reverse their natural po- 

 sition. The shell is beneath, and the underside of the 

 foot upwards ; this is exhibited by the LymiHEa, 

 Planorbis, Paludina, and others. This last mode of lo- 

 comotion is but rare in the acephalous molluscs, though 

 one or two instances have been given upon good au- 

 thority ; something analogous may be supposed to exist 

 in the Nuc^tla. Judging from the arrangement of the 



foot, the movement of this class is often totally confined 

 to the inconsiderable expansion of their valves. With 

 regard to locomotion in molluscs, generally, the 

 first circumstance to be considered is the naturae 

 position of the animal while in a state of repose, for 

 it is only at that moment their food is supplied to 

 to them in the water, particularly when the mouth 

 of the animal does not possess extensible tubes ; 

 the expansion of their valves in a greater or 

 smaller degree governing the quantum of food, or the 

 convenience of receiving it : we have already ex- 

 plained that this is constantly effected without any 

 effort on the animal's part, by means of the ligament 

 or spring connecting the valves ; the act of shutting 

 the valves being effected by the adductor muscles act- 

 ing as antagonists against this ligamentous fastening. 

 Leach and some other malacologists have imagined 

 that a part of the central adductor muscle was formed 

 of an elastic substance in the oyster and other bi- 

 valves ; but it is not confirmed by subsequent observa- 

 tions, and therefore seems doubtful. The family of 

 the Palliobranchiata, or such as have their branchise 

 situated on the internal part of the mouth, or the pal- 

 lium, exhibit examples of genera in which, instead of a 

 ligament, the two valves are united at their summit 

 by a long elastic tube affixed to sub-marine bodies, 

 being in some cases slightly contractile ; the animal, 

 however, enjoys no other movement than the opening- 

 and shutting of its valves, like other acephala. In the 

 species of molluscs fixed by their shells or by a tube, 

 these being the only motion possible, a very simple 

 organisation of foot is either requisite or visible. In 

 the other species more locomotion exists, though still 

 to a very limited extent ; they are not, in fact, fixed 

 by the immediate contact of the shell with another 

 object, but by fibres or a byssus ; and though they 

 have the power of fastening this dried muscle, and as 

 it were anchoring themselves to a solid place by its 

 assistance, yet they cannot voluntarily detach them- 

 selves from that fastening; the method employed to 

 affix themselves is by means of the very long- exten- 

 sible foot, particularly characteristic in the byssiferous 

 molluscs. 



The Tridacna, Area, &c , have also the faculty of 

 attaching themselves to solid bodies by an agglutina- 

 tion of the foot ; but, unlike the byssus, it is in mass, 

 and not in separate fibres. In by far the greater num- 

 ber of molluscs without heads, the power of locomotion 

 is more extended ; the use of the foot in its proper 

 office is, that of aiding their removal from place to 

 place ;some merely exercise it in ascending or descend- 

 ing the hole in which they have fixed their location, 

 whether in wood, stone, mud, or sand ; their foot 

 being attached more anteriorly than in the other spe- 

 cies, is more readily extended, and takes its point of 

 rest at the bottom of the cell : this always takes place 

 in the Pyloridea, whether tubular or not. All other 

 bivalve molluscs, although they often live more or 

 less buried in the mud or sand, can always quit it 

 voluntarily, or move to any other spot ; some perform 

 locomotion by means of leaping, the foot serving as a 

 spring, and acting in the same way by its contraction 

 and sudden relaxation ; from this mode of motion, 

 the name of Subsilentia, or leapers, was given by Poll 

 to the headless molluscs, but evidently erroneously ; 

 for if the greater part of the animals of this family of 

 shells possess that power, the Arks and others do not, 

 but are permanently fixed, and appear really to climb 

 with their foot. 



