Burrowing and Stationary Mollusca. 361 



fine sand to work in, the ikfyades coarse gravel, and the 

 Cardia are often found in sludge. Some dig scarcely deeper 

 than just to cover and conceal themselves; others penetrate 

 to a depth of one or even two feet, ascending and descending 

 in the furrow with a velocity rather surprising for creatures 

 so habitually sluggish. They effect these motions by varying 

 at will the length and form of the foot, the same organ with 

 which they had in the first instance dug their furrow. When 

 the animal would burrow, it projects and elongates the foot, 

 distending it until every part of it, except the point, appears 

 semitransparent. Directing its point downward, it insinu- 

 ates it into the sand until it is nearly buried. A circular 

 motion is now given to the shell, by which its anterior point 

 is quickly brought nearly into contact with the foot, and 

 immediately returned to its former situation. It thus moves 

 on the foot, as on a fulcrum, with a see-saw motion. The 

 foot, which had been partially retracted, is again gradually 

 projected as far as possible into the sand, when the circular 

 motion of the shell is repeated. When the animal is moder- 

 ately active, the strokes follow each other at intervals of 

 twenty or thirty seconds. The apparent progress is at first 

 but small ; the shell, which is raised on its edge at the mid- 

 dle of the stroke, falling back on its side at the end of it : 

 but, when the shell is buried so far as to be supported on its 

 edge, it advances more rapidly, sinking visibly at every 

 stroke, till nothing but the extremity of the siphon 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 is likewise perforated to near its 

 point with a tube, which, opening just within the mouth, con- 

 veys water to distend 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 communicate 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, at least when full grown, 

 attempt to rebury themselves. But there is a species of 

 Gasteropode 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 waved whelk (Z?uccinum 



