HOW THE ROCK IS PERFORATED. 489 



mens immediately protrude their siphons, and explore 

 the surrounding bottom with them in a remarkably 

 worm-like manner. The branchial currents com- 

 mence instantly, and never cease unless the creatures 

 are disturbed. The force and volume of these cur- 

 rents are quite marvellous to those who witness them 

 for the first time. The inhalant orifice is trumpet- 

 shaped, and guarded with cirrhi ; the exhalant is a 

 little contracted, and projects beyond the other. 



The manner in which the stone- borer?, " Stein- 

 bohrers" as the Germans call them, excavate their 

 abodes, has long been a subject of interesting debate, 

 and is even yet by no means satisfactorily settled. So 

 completely was this part of their history unintelligible 

 to the earlier naturalists, that Rondeletius was abso- 

 lutely led to suppose, that the sea-water lodged in 

 cavities in the rocks was itself converted into Pholades 

 and other saxicavous mollusca ! * and even the more 

 philosophical Reaumur was driven to the belief that 

 they entered the earth while it was yet in a soft state, 

 which afterwards hardened by degrees around them. 

 Various other explanations of this phenomenon have 

 subsequently been at different times suggested, but 

 scarcely with better success. Some have supposed 

 that the creatures bore by the aid of a solvent liquor ; 

 others that their excavations are effected by file-like 

 rasps, situated upon the exterior of the shell, and 

 worked by its semi-rotatory movements, produced by 

 appropriate muscles. Of late, various other theories 



* " Ego crediderim in saxorum cavernulis, vel vi vel natura 

 factis aquas marinae appulsu procreari, atque in concham verti, 

 qua3 cavitatis sive foraminis figuram servat." 



Y 5 



