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LETTER IX. 



THE BURROWING AND STATIONARY MOLLUSCA. 



My last letter concluded with some account of the loco- 

 motion of certain of the Bivalve Mollusca, but you must 

 now be told that the great majority of this class reside 

 habitually in furrows which they have excavated in the 

 gravel, sand, or mud, where they live concealed from their 

 foes, and protected against the rudeness of the sea, which 

 would otherwise cast them helpless on the shore, as you 

 must have noticed is frequently the fact when the storm has 

 been sufficiently severe to remove the under-soil. Then is 

 the time for the Conchologist to take his walk ; and he will 

 do well to guide himself in his search for the objects of his 

 study by noticing where the crows and sea-birds are prin- 

 cipally congregated. The Solenes and Telhnida) prefer fine 

 sand to work in, the Myades and Lutrariae 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 mo- 

 tions 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 semi-transparent. Directing its 

 point downwards, it insinuates 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 re- 

 tracted, is again gradually projected as far as possible into 

 the sand, when the circular motion of the shell is repeated. 

 When the animal is moderately 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 middle of the stroke, falling back 

 on its side at the end of it ; but, when the shell is buried 



