140 BURROWING BIVALVES. 



so far as to be supported oil its edge it advances more ra- 

 pidly, sinking visibly at every stroke, till nothing but the 

 extremity of the siphons can be perceived above the sand.* 

 These motions of the foot and shell are effected by two 

 pairs of muscles, which arise from the shell and are inserted 

 into the foot, which they embrace ; but this organ, in some 

 genera, is likewise perforated to near its point with a tube, 

 which, opening just within the mouth, conveys water to dis- 

 tend and stiffen it. In some of the largest species, as in 

 Cyprina islandica, a transverse section of the foot shows a 

 single chain of pores along its whole length, which commu- 

 nicate with this tube, and transmit the water to the cellular 

 portion of the foot ; and, when thus distended, a viscid 

 matter is secreted from its surface, which, by agglutinating 

 the sand around it, fixes it more firmly, and thus augments 

 the force of the stroke. 



These burrowing tribes never, I believe, voluntarily quit 

 their cells ; and, if torn from them by the action of a stormy 

 sea or any other cause, they rarely, when full grown, attempt 

 to re-bury themselves. f But there is a species of Gaste- 

 ropod, which, generally living on the surface, has yet the 

 power to burrow, and does so, it would appear, habitually 

 when in search of prey. This is the Whelk (Buccinum 

 undatum), so common on our coasts. " As in the Bivalves 

 inhabiting sand, its foot is the instrument of penetration ; 

 and, like them, it has the power of distending this organ to 



* "By the action of this foot these animals can bore with great facility 

 in the sand, where some are found at considerable depth. They can like- 

 wise accomplish a quick progression, by using it as a hook, or pushing them- 

 selves forward by its means ; they also swim on the surface of the water, by 

 expanding it into a concave dish, and climb perpendicular surfaces by fixing 

 its extremity like a sucker. Some species have the power of secreting air 

 into two sacs of the mantle, attached to the excretory organs, by which their 

 specific gravity is diminished ; and they readily change their situation at the 

 ebb and flow of the tides." GARNER in Charlesw. Mag. Nat. Hist. iii. 128. 



f " If the Solen be taken out of its hole and placed upon the sand, it im- 

 mediately prepares to re-bury itself. It stretches out its foot to full length, 

 and then bends it so as to use the extremity as a kind of auger. When the 

 end has sunk into the sand, it draws up its shell, which, first oblique, and 

 afterwards perpendicular, soon becomes immersed and rapidly disappears. 

 M. Deshayes, during his Algerian researches, observed a remarkable instinct 

 of Solen marginatus to swim, when desirous of changing its locality. When 

 it finds itself on ground too hard to be penetrated by its foot, it fills the 

 cavity of its mantle with water, and then contracting, and closing exactly at 

 the same time its siphonal orifices, elongates its foot ; then re-contracting 

 that organ, it ejects the water with force from the tubes, and thus propels 

 itself after the manner of a cuttle-fish, for a foot or two forward. Then, if it 

 finds the surface favourable, it bores and buries itself; but if not, makes 

 another leap to try its chance anew." FORBES andHANLEY, Brit. Mollvsca, 

 i. 245. 



