160 THE BORINfi MOLLUSCA. 



the mantle, vvliere it swells into a thick convex mass of 

 interlaced and chiefly transverse muscles.' Surely this 

 powerful muscular apparatus has some important function 

 to perform, — not to secrete a solvent, but to assist by its 

 mechanical acrencv in the work of excavation. 



" We now see the boring instrument complete in all its 

 parts ; and a more efficient apparatus could not be devised. 

 Supplied with this flinty armature, the soft fleshy foot of 

 Pholas and Teredo, adhering to the substance to be reduced, 

 and aided by the edges of the mantle, cuts with equal 

 facility into wood, shale, chalk, and the various other bodies 

 into which these mollusks burrow. Patella excavates in the 

 same way. The mode is somewhat varied in Gastrochaena 

 and Saxicava ; they firmly attach themselves by the byssus 

 to the rock, then bring into contact with it the armed and 

 thickened portion of the mantle, thus enabling the interlaced 

 muscles of which it is composed to work with as much ef- 

 fect as those in the broad adhesive foot and mantle of Pholas 

 and Teredo. 



" In none of these species is much rotatory motion re- 

 quired. In Pholas and Teredo, little more than the mere 

 contraction of the cutting-surface is sufficient ; each portion 

 of the foot and mantle, which together nearly fill up the 

 bottom of the excavation, acting immediately on the sub- 

 stance with which it is in contact. The same thing takes 

 place in Patella, which evidently does not rotate, for the 

 burrows are elliptical, like the animal, and fit with great 

 accuracy the marginal indentures of the shell. But the 

 cutting disk of Saxicava and Gastrochaena being narrower 

 than the burrows, these species must, at intervals, move a 

 little from side to side, anchoring themselves afresh by the 

 byssus whenever they shift their position. In all, however, 

 the same vermicular contraction of the parts, observed by 

 Sir Everard Home in the foot or ' proboscis ' of Teredo, 

 will be required to remove the substances into which these 

 animals bore. 



" Thus this perplexing subject is simplified ; and judging 

 from analogy, there can be little doubt that all the boring 

 mollusks excavate in the same manner : none by the rasping 

 or cutting of their valves, — none by a solvent, — none by 

 ciliary currents. We should, therefore, be inclined to 

 doubt that any of the Acephala bore, which are not pro- 

 vided with either the broad adhesive foot, or with the 

 thickened mantle united in front. Venaerupis perforans 

 may be, perhaps, cited as an exception to this rule ; but it 

 is doubtful whether it ever bores. On the coast of North- 



