LAMELLIBRANCHIATA. 519 



adaptation for the fanction of excavating wood. It indicates a mind 

 unfitted for physiological discovery to deny this adaptation, because 

 the exterior of the valves be sometimes coated by a dried layer of 

 the abundant mucus which is exuded from the pedial aperture 

 during the active movements of the borer. The rasp-like exterior of 

 the shell of the Pliolas crispata, with the modifications of the ad- 

 ductors and their fulcral apophyses, in like manner, suggests the 

 rasping rotatory action by which the valves may produce or aid in 

 producing the burrows in the rocks in which the piddocks conceal 

 themselves. To deny this use of the Pholas-shell, because the shell 

 of some other rock-boring bivalves is smooth, is a another sign of a 

 narrow mind. There are, doubtless, other modes of boring besides 

 the shell-action ; but the recognition of any such need not involve a 

 negation of every mode but the one so recognised. 



Mr. Osier* has advocated the hypothesis of a chemical solvent as 

 the boring agent ; but such solvent has not been demonstrated ; and 

 the necessity of its being applied in currents of water to such cal- 

 careous rocks on which it could alone operate, with the liability of 

 the shell of the animal secreting the solvent to be affected thereby, 

 have been insuperable obstacles to the acceptance of the hypothesis. 

 Mr. Garner f has called attention to the ciliary currents generated 

 by the extensive surface of ciliated epithelium in the lamellibranchs 

 as probable aids to the rasping action of the valves : and since this 

 demonstrated and constantly acting dynamic causes as unceasing a 

 current of water in the holes of the borers, the non-extension of such 

 current between the shell and the rock, where they may be in close 

 contact, is no argument against the influence of the current in the 

 rest of the hole, and especially at the line where it is opposed by such 

 contact. The ingenious idea of the ciliary action as an accessory 

 power in boring may, therefore, be accepted, from its universal ap- 

 plicability, and is certainly worthy of notice. 



More than twenty years ago I suggested the same kind of instru- 

 ment as applied to boring in rock, which had been recognised as the 

 one used for burrowing in sand. The anatomy of the ClavagellaJ 

 offered many points highly suggestive of the inadequacy of the hypo- 

 theses of the burrowing-agents promulgated at the time when I first 

 had the opportunity of dissecting that lithodomous bivalve ; and its 

 structure indicated a power that had not been previously suspected 

 in rock-borers. In the first place, it was evident that the valves 

 could not act, as they do in Teredo and Pholas; for the terminal 



* CCCXVII. p. 270. t CCCXIV. X CCCXX. p. 269. 



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