THE ROCK-BORERS. 203 



use of hot water will generally accomplish the desired end. Animal glue or gum-water cannot 

 be recommended. 



Every visitor to the sea-shore has observed rocks drilled with innumerable holes, almost 

 :as though by art. A few good blows with a stout hammer on the chisel-head serve to 

 ;split off a great slice of the coarse red sandstone. The holes run through its substance, 

 but they are all empty, or filled only with the black foetid mud which the sea has de- 

 posited in their cavities. These are too superficial; they are all deserted; the stone lies too 

 high above low- water mark ; we must seek a lower level. Try here, where the lowest 

 spring- tide only just leaves the rocks bare. See ! now we have uncovered the operators. 

 Here lie snugly ensconced within the tubular perforations, great mollusca, with ample ivory- 

 like shells, which yet cannot half contain the whiter flesh of their ampler bodies, and the 

 long stout yellow siphons that project from one extremity, reaching far up the hole towards 

 the surface of the rock. 



We lift one from its cavity, all helpless and unresisting, yet manifesting its indignation 

 ;at the untimely disturbance by successive spasmodic contractions of those rough yellow 

 siphons, each accompanied with a forcible jet d'eau, a polite squirt of sea-water into our 

 faces ; while at each contraction in length, the base swells out till the compressed valves 

 of the sharp shell threaten to pierce through its substance. 



Strange as it seems, these animals have bored these holes in the stone, and they are 

 capable of boring in far harder rock than this, even in compact limestone. The actual 

 mode in which this operation is performed long puzzled philosophers. Some maintained 

 that the animal seci'eted an acid which had the power of dissolving not only various kinds 

 of stone, but also wood, amber, wax, and other substances in which the excavations are 

 occasionally made. But it is hard to imagine a solvent of substances so various, and to 

 know how the animal's own shells were preserved from its action, while, confessedly, no 

 such acid had ever been detected by the most careful tests. Others maintain that the 

 rough points which stud the shell enable it to serve as a rasp, which the animal, by rotating 

 on its axis, uses to wear away the stone or other material ; but it was difficult to understand 

 how it was that the shell itself was not worn away in the abrasion. 



Actual observation in the aquarium has, however, proved that the second hypothesis 

 is the true one. M. Cailliaud in France, and Mr. Robertson in England, have demonstrated 

 ihat the Pholas uses its shell as a rasp, wearing away the stone with the asperities with 

 which the anterior parts of the valves are furnished. Between these gentlemen a somewhat 

 liot contention was maintained for the honour of priority in this valuable discovery. M. 

 Cailliaud himself used the valves of the dead shell, and imitating the natural conditions 

 .as well as he could, actually bored an imitative hole, by making them rotate. Mr. Robertson 

 at Brighton exhibited to the public living Pholades in the act of boring in masses of chalk. 

 He describes it as " a living combination of three instruments, viz., a hydraulic apparatus, 

 a rasp, and a syringe." But the first and last of these powers can be considered only as 

 an accessory to the removing of the detritus out of the way when once the hole was bored, 

 the rasp being the real power. If you examine these living shells you will see that the 

 fore part, where the foot protrudes, is set with stony points arranged in transverse and 

 longitudinal rows ; the former being the result of elevated ridges radiating from the hinge, 



