212 LECTURES ON 



for a time, that the species may become properly diffused — lie now 

 seeks a home of work and rest. He swims to the bottom, and alights 

 on the oyster bed. Admiring the many caves already disintegrated 

 by the action of other living creatures, he chooses one for his abode. 

 If no such cave exists, he shelters himself behind a corner and sets 

 to work to make one. Fancy a man on a desert island planting him- 

 self against a rock, with a deliberate purpose of hollowing out a cave 

 to dwell in by rubbing with his hand or foot ! And yet this is what 

 the young Gastrochaena proposes to himself, when the "days of 

 helpless infancy" are hardly ended. And he does it. Those who 

 have witnessed the polishers of fancy marbles smoothing hard stones 

 by rubbing them in their hands may form some idea hoiv the Gastro- 

 chaena does it. A soft living tissue is always stronger than a hard 

 dead one. The coral-polype weathering the stormy breakers proves 

 how a feeble vital force may resist a mighty energy which is only 

 mechanical. Long as the. boring bivalves have been known and 

 studied, their mode of operation is still matter of conjecture. But 

 the study of our oyster burrows threw some light on the dark places. 

 If the account here given be incorrect, let naturalists, with the marine 

 aquaria that are to be, correct it. It used to be supposed that they 

 dissolved away the rock by secreting acid; but not only was the said 

 acid never detected among the - secretions of the body, it was also 

 suggested that the same solvent which destroyed the calx of the cavity 

 would also destroy that of the shell itself. The hypothesis, however, 

 answered sufficiently well so long as the borers were only found in 

 shells or limestone rocks. But at last they were found burrowing in 

 hard, primitive silex, which no acid could touch.* It was then sug- 

 gested that the animal spun itself round and round, like a tetotum, or 

 backwards and forwards, like an awl, and so filed away the matrix. 

 The delicate imbrications on the valves of the Pholas tribe seemed to 

 favor this view; but even here we should expect to find the fragile 

 file worn out by rubbing, which is not the case, the points in well- 

 conditioned specimens being as little injured as the remainder of the 

 shell. Moreover, it is the open, gaping portion of the shell that is 

 turned first towards the resisting medium. Besides, even if the 

 Pholads were proved thus to bore, the Gastrochamids are generally 

 devoid of sculpture, except lines of growth; the Lithophagi are often 

 so twisted that they cannot move round in their hole's; and the bur- 

 rows of many of the Petricolidoa are heart-shaped, like the valves. If 

 not the only agent of disintegration, I am convinced that in all cases 

 the foot is the principal weapon of attack. This is known to be often 

 strengthened by silicious particles, and I found its dried remains, as 

 hard as horn, in adult burrows of (I think) every species of borer 

 found in the Spondylus. This organ, which forms the principal part 

 of the solid substance of the animal in all the burrowing and leaping 

 tribes, is variously modified either to scoop out soft sand with 



$ ° A very beautiful specimen of Pholas in its flinty home will be found in the State 

 Museum, at Albany. 



