J. Rqfe — On Lithodomous Perforations, 9 



that " when the animal is merely gliding along and not using the 

 tongue, a little bubble of fluid is constantly, at short intervals, 

 emitted from the mouth, whicli partially opens for the purpose. I 

 have repeatedly saturated strijis of litmus paper in this fluid, both 

 at the head and all along the foot, but have entirely failed to detect 

 any pink colouring denoting the presence of an acid." M. Bouchard, 

 however, found that if he removed the snail whilst in the act of 

 perforating the limestone, the exudation from its mouth did redden 

 litmus paper, showing that the secretion was then acid. Professor 

 Huxley states that the commonly called palate of the Mollusca, 

 which he names the odontophore, " consists essentially of a carti- 

 laginous strap which bears a long series of transversely disposed 

 teeth. The ends of the strap are connected with muscles attached 

 to the upper and lower surface of the hinder extremities of the 

 cartilaginous cushions, and these muscles, by their alternate con- 

 tractions, cause the toothed strap to work backwards and forwards 

 over the end of the pulley formed by its anterior end. The strap 

 consequently acts after the fashion of a chain-saw (or rather of a rasp) 



upon any substance to which it is applied Besides the 



chain-saw like motion of the strap, the odontophore may be capable 

 of a licking or scraping motion as a whole." ^ 



From this it appears most probable that to the odontophore of the 

 snail we must look for the tool by which this work is carried on, 

 and, from the above observation of M. Bouchard, to all appearance 

 with the aid of an acid which may possibly be secreted by the salivary 

 glands, excited by the alternate action of the above strap, analogous 

 to the action of mastication on our salivary glands ; and although 

 our saliva may not be acid, the effect of the action may be the same, 

 though the secretions differ. In our case the acid necessary is 

 secreted with the gastric fluid during digestion ; in that of the snail 

 the acid solvent would be secreted as required to assist the action on 

 the limestone. But supposing these surmises to be incorrect, and 

 that there is no chemical action, the mechanical effect of the file-like 

 teeth would probably suffice for the purpose, though not so rapidly. 

 The action of fine sand-paper on marble will illustrate this, as the 

 odontophore acts in the same manner. 



It has also been suggested that these holes were originally made 

 by the Pholas, and that, after the elevation of the land, they were 

 enlarged by snails ; but I cannot see anything to justify the suppo- 

 sition, as, if it is admitted that snails could enlarge the holes and 

 make fresh chambers in them, surely they must have power to 

 commence them, and the action of the Pholas would be quite unne- 

 cessary. But a difference in the habits and nature of the Pholas and 

 snail must be considered, which in itself appears fatal to the suppo- 

 sition of joint action. The burrows of the Pholas are described by 

 Dr. S. P. Woodward^ as vertical and quite symmetrical, whilst those 

 in question are never either the one or the other. The Pholas, being 

 aquatic and requiring water for breathing, makes its hole down- 

 wards, so as to retain some moisture when the tide recedes ; whilst 

 ^ Elements of Comparative Anatomy, p. 36. ' Manual of the Mollusca, p. 327. 



