242 MANUAL OF THE MOLLUSCA. 



chalk, shale, clay, soft sandstone and sandy marl, and decomposing //wm^;* 

 it has also been found boring in the peat of submarine forests, in wax, and in 

 amber.f It is obvious that these substances can only be perforated alike 

 by mechanical means ; either by the foot or by the valves, or both together, 

 as in the burrowing shellfish. The ])holas shell is rough, like a file, and 

 sufficiently hard to abrade limestone ; and the animal is able to turn from 

 side to side, or even quite round in its cell, the interior of which is often 

 annulated with furrows made by the spines on the front of the valves. The 

 foot of the pholas is very large, filling the great anterior opening of the 

 valves ; that of the ship-worm is smaller, but surrounded with a thick collar, 

 formed by the edges of the mantle, and both are armed with a strong epithe- 

 lium. The foot appears to be a more efficient instrument than the shell in 

 one respect, inasmuch as its surface may be renewed as fast as it is worn 

 away.;}: {Hancock.) 



The mechanical explanation becomes more difficult in the case of another 

 set of shells, lithodomus, gastrochcena, saxicava, and ungulina, which bore 

 only into calcarious rocks, and attack the hardest marble, and still harder 

 shells (fig. 25, p. 42). In these the valves can render no assistance, as they 

 are smooth, and covered with epidermis; neither does the foot help, being 

 small and finger-like, and not applied to the end of the burrow. Their power 

 of movement also is extremely limited, their cells not being cylindrical, whilst 

 one of them, saxicava, is fixed in its crypt by a hyssus. These shell fish have 

 been supposed to dissolve the rock by chemical means [Beshayes), or else to 

 wear it away with the thickened anterior margins of the mantle. {Hancock.) § 



The holes of the lithodomi often serve to shelter other animals after the 



* There is a specimen from the coast of France, in the Brit. Museum. 



+ Highgate resin, in the cabinet of Mr. Bowerbank. 



X The final polish to some steel goods is said to be given by the hands of work- 

 women. In Carlisle Castle they point to the rude impression of a hand on the 

 dungeon wall, as the work of Fergus M'Ivor, in the two years of his solitary im- 

 prisonment. 



§ All attempts to detect the presence of an acid secretion have hitherto failed, as 

 might be expected ; for the hypothesis of an acid solvent supposes only a very feeble 

 but continuous action, such as in nature always works out the greatest results in the 

 end. See Liebig's Organic Chemistry, and Dumas and Boussingault on the "Balance 

 of Organic Nature." Intimately connected with this question are several other 

 phenomena; the removal of portions of the interior of univalves, by the animal 

 itself, as in the genera Conus, Auricula, and Nerita (fig. 24, p. 40) ; the perforation of 

 shells by the tongues of the carnivorous gasteropods :and the formation of holes in 

 wood and limestone by limpets. Some facts in surgery also illustrate this subject, 

 (1) dead bone is removed when granulations grow into contact with it : (2) if a hole is 

 bored in a bone and an ivory peg driven into it, and covered up, so much of the peg as 

 is imbedded ia the bone will be removed. {Paget.) The " absorption" of the fangs 

 of milk-teeth, previous to shedding, is well-known. In these cases the removal of the 

 bone earth is effected without the development of an acid, or other disturbance of the 

 neutral condition of the circulating fluid. 



