394 MANUAL OF THE MOLLUSCA. 



The boring shell-fisli have been distinguished from the mere 

 burro wers, perhaps without sufficient reason, for they are found 

 in substances of every degree of hardness, from soft mud to 

 compact limestone, and the method employed is probably the 

 same.* 



The means by which bivalves perforate stone and timber has 

 been the subject of much inquiry, both on account of its phy- 

 siological interest, and the desire to obtain some remedy for 

 the injuries done to ships, and piers, and breakwaters. The 

 ship-worm [teredo) and some allied genera, perforate timber 

 only ; whilst the pholas bores into a varietj'- of materials, such 

 as chalk, shale, clay, soft sandstone and sandy marl, and 

 decomposing gneiss ;t it has also been found boring in the peat 

 of submarine forests, in wax, and in amber. :j: 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 pholas 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 valyes ; 

 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 epithelium. 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 awav.§ (Han- 

 cock.) 



The mechanical explanation becomes more difficult in the 

 case of another set of shells, lithodomus, gastrochcena, saxicava, 

 and ungulina, which bore only into calcareous rocks, and attack 

 the hardest marble, and still harder shells. (Fig. 25, p. 34). 

 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 



* See the admirable memoir bj' Mr. Albany Hancock, in the An. Nat. Hist, for 

 October, 1848. 



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



t Highgate resin, in the cabinet of Mr. Boworbank. 



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

 women. In Carlisle Castle they point to the rnde impress'on of a hand on the 

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

 prisonment. 



