BURROWING MOLLUSCA. 141 



a size nearly, if not quite, equal to that of the shell." 

 " The moderately distended foot can scarcely be retracted 

 within the margin of the shell ; and when fully injected, it 

 is elastic, and of a very large size. The cavity which it 

 opens into the sand is therefore fully adequate to receive 

 the shell, which is drawn down into it by the contraction 

 of the muscle of the spire. From the attachment of this 

 muscle the spire is the part more directly acted upon, and 

 which is depressed in the greatest degree. Hence the notch 

 is always uppermost ; and the Buccinum, when completely 

 buried, is enabled to communicate with the water by its 

 respiratory siphon." The habits of the Cassides, or Helmet- 

 shells, of Cymbium, of the Natieae, Bullae, and of several 

 others amongst the carnivorous tribes of Gasteropods are 

 similar to those of the Whelk, and they have a foot of similar 

 structure. 



The locomotion of the burrowers is then of a restricted 

 character, confined chiefly to partial movements in the fur- 

 row, but they are not prevented by a physical hinderance 

 from the ability of removal, if a strong necessity should urge 

 it. Another tribe, of greater interest, unable to burrow, 

 and yet from their littoral habits much exposed to the sea, 

 willingly secure their ease and safety by a sacrifice of liberty.* 

 Foremost amongst these are the genera which moor them- 

 selves to the rocks by what the vulgar call the beard of the 

 fish, and the learned its byssus. Such are the Mussels, 

 the Hammer-shells, the Pinnae, the Pectens, and a few others 

 of less common note ; but these others of lesser note illus- 

 trate a very general rule observable everywhere on the in- 

 troduction of any new mode in the economy of animals, that 

 the mode is never done abruptly. So here we find mollusks 

 that make, as it were, an essay of the utility of a byssus 

 before the resolve to become anchorites is taken up by others. 

 The pretty family, Kelliadae, spin occasionally a feeble bys- 

 sus, and of few threads, by which they attach themselves 

 temporarily only, for they can unfix themselves again at 

 pleasure, and do so repeatedly during the course of their 

 lives. f And the Tapes pulastra, which is usually free, for 



* " Travelling is not good for us, we travel so seldom." — " How much 

 more dignified leisure hath a mussel, glued to his impassable rocky limit, 

 two incites square ! He hears the tide roll over him backwards and forwards, 

 twice a day (as the Salisbury long coach goes and returns in eight and forty 

 hours), but knows better than to take an outside place a-top on 't. He is 

 the owl of the sea, Minerva's fish — the fish of wisdom." — Lamb's Letters, 

 Edited by Talfourd, i. 319. 



f Forbes and Hanley, Brit. Mollusca, ii. 70, 78, 79. 



