THE BORING MOLLUSCA. 157 



force, the failure of the experiment cannot be considered to 

 militate very strongly against the only inference to be drawn 

 from the facts. And it may be observed that, where the 

 Lithophaga happen to be lodged in situations which afford 

 them sufficient room and shelter, they make no attempt to 

 enlarge their habitation. Thus Saxicava praecisa (Fig. 28 b) 

 is more frequently found among groups of Serpulae, or in 

 the roots of sea-weed, than in a hole excavated by its own 

 efforts ; and Air. Osier has obtained full-grown specimens 

 of Saxicava arctica (Fig. 28, c), attached by the byssus to a 

 Pecten. It may therefore be presumed that the solvent is 

 secreted only when its agency is required ; and this would 

 sufficiently explain why a free acid cannot be detected in 

 the animal by any chemical tests. 



The views of Mr. Osier were eagerly adopted by British 

 naturalists, at least, and they remained unchallenged for 

 several years, perhaps from an unwillingness to re-agitate 

 and unsettle a question that had long fretted the inquirer 

 by its uncertain and debatable character. * Mr. Garner, 

 without any formal attempt to expugn them, preferred a 

 very different explanation ; for, availing himself of the modern 

 discovery of the existence of vibratile cilia on many of the 

 surfaces of animals, he almost assumed that the currents 

 of water produced by the cilia of the mollusca in question 

 were sufficient to work out their excavations, f The in- 

 sufficiency of this cause seemed always to me almost self- 

 apparent, and that it is so has been amply proved by Mr. A. 

 Hancock ; who has, with no little ability, shown the equal 

 invalidity and erroneousness of all previous explanations. 

 He reviews, in an interesting manner, the arguments and 

 facts which have been adduced to prove that the Teredo 

 bores in the manner of an auger, and he finds ample proof 

 of the contrary in the shape and habits of the animal : he 

 shows us that the Pholas has not the amount of rotatory 



* " And, indeed, this is an error, which is very natural to men's minds : 

 they love not a long and a tedious doubting, though it brings them at last to 

 a real certainty ; but they choose rather to conclude presently, than to be 

 long in suspense, though to better purpose." — Sprat's Hist. Roy. Soc. 32. 



T " It appears, then, that the mechanical apparatus of the different boring 

 animals of this class is insufficient to account for their power of excavation ; 

 and we must attribute it principally to the action of the ciliated foot and 

 tentacles causing a never-ceasing vortex at the inferior extremity of the cell. 

 In some of these animals, too, the body is much produced, having the tube 

 of its mantle garnished with its continuous branchiae, the cilia of which must 

 give great force to the rushing column of water. If any species make use 

 of its valves as adjutory, it would be the Teredo, which attacks the hard 

 planks of ships."— Garner in Charlesw. Mug. IV. Hist. ill 300, 3* >1 . 



