THE BORING AIOLLUSCA. 151 



had for long divided less imaginative naturalists, — the one, 

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

 they excrete ; the other, that they do so by processes, or 

 hard portions of the shell, worked by its semi-rotatory mo- 

 tions, and regulated by appropriate muscles. Of late, other 

 two theories have been propounded : the borings have been 

 ascribed, by an ingenious author, to the action of currents of 

 water, directed against the parts to be worn away, by the 

 ceaseless play of cilia on the animal's body, these currents 

 acting not so much by their force, as by their constant and 

 long-continued impulse, just as the drop from the eave will 

 in time wear a basin in the stone floor underneath. The 

 other theory ascribes the whole works of the whole tribe to 

 the animal itself operating on the wood or rock with an 

 organ fit and fitted for the purpose. 



The mechanical theory seems to have been suggested, in 

 the first instance, by a false view of the valves of the 

 Teredo, which their form misled the earlier naturalists to 

 believe were the teeth of the animal, and that with them the 

 animal eat its way into the wood.* When this mistake of 

 office was discovered, the organs were construed to be au- 

 gers ; and the other boring mollusks, it was affirmed, had 

 organs adapted for a similar purpose either in the spinous 

 processes or in harder and thicker margins in front, or, as 

 in the instance of the Lithodomus, in the shape of the shell 

 itself, which needed only to be put into a rotatory and for- 

 ward motion by the muscular efforts of the mollusk. The 

 fact of there being such a rotation of the shell was deemed to 

 be proved by the circular striae which some naturalists had 

 observed on the walls of the cells of our common Pholades. 



The chemists, on their part, were driven to a hypo- 

 thetical acid by the many objections which seemed to render 

 any mechanical explanation untenable. There was no pro- 

 portion between the creature's physical powers and the 

 results produced ; the substance operated upon was, in 

 many cases, harder than the shell, and more likely to wear 

 away its processes and asperities than itself to be perfo- 

 rated ; and yet the shell remained intact, and there were no 

 appearances on its parts or surface to indicate that it had 

 been used either as a rasping or boring instrument, the very 

 skin covering the valves remaining uninjured. It was even 

 said, that the form and size of the cell made a rotatory mo- 

 tion of the animal in it impossible, as surely was the case 



* Home's Com}). Anat. i. 377. Dclle Chiaic defines the Teredo—" Ani- 

 mal— anterius maxillis lignum terebrans." — Anim, Naji.s. Vert. iv. 32. 



