276 



THE NATtJRALIST. 



mining the rocks of our slioreSj-as completely as tlie most scientific system 

 of engineering could do . 



What are tlie engineers ? and what wonderful instrument do they employ? 

 The engineers, are various kinds of bivalve mollusca, and the instrument 

 used is a portion of their own body. In fact all these strange holes, and 

 burrows are the work of animals with soft gelatinous bodies, protected by 

 shells which, although hard, are at the same time very brittle. It is no won- 

 der then, that admiration and curiosity has long been excited by these 

 strange rock-boring animals, and that their mode of working has been a 

 favourite bone of contention between rival naturalists. 



Four theories for accounting for this excavation by rock-boring mollusca 

 have at different times found favour with naturalists, namely, that the exca- 

 vations were efiected : — 1. By the aid of a chemical solvent ; or, 2. By 

 absorption ; or, 3. By the assistance of ciliary currents ; or, 4. By the 

 rotation of the animal. 



The great difficulty connected with the first, that of a chemical solvent, 

 is that these mollusca perforate all kinds of substances and it would be 

 manifestly impossible to find an acid wliich would act on almost every Imovm 

 rock, as also on wood and wax ; and yet would have no eff'ect on the valves of 

 tlie shell. The absorption theory has never found much favour among natural- 

 ists, it is therefore unnecessary to dwell upon it. The third theory, that of 

 ciliary currents, is discountenanced by the circumstance that very few if 

 any of the rock-boring mollusca possess ciKa, and it could therefore at the 

 utmost apply to only a small number of them, and moreover it fails to 

 explain all the phenomena exhibited. 



The theory of rotatory motion, or of a rotatory motion combined in 

 some instances with a process of rasping, is that now generally received. 

 Independently however, of a rotatory motion being necessary, it is also required 

 that there should be some means of carrying away the particles of wood or 

 stone removed. All these various requirements are beautifully provided for in 

 these mollusca. 



The excavation of the burrow is chiefly accomplished by the foot^ 

 assisted to a small extent in some instances by the minute sharp teeth on 

 the shell, as is clearly shown by the longtitudinal furrows on the sides 

 of the perforation. In cases however, where the valves of the shell 

 are perfectly smooth, the foot is the sole excavating organ. Let us now see 

 how the foot is set in motion, and by what m.eans the particles rubbed off 

 are crot rid. of. 



