coNcmFEEA. 395 



its ciypt hy B.iyssus, These shellfish have been supposed to 

 dissolve the ruck by chemical means (Deshayes), or else to 

 wear it away with the thickened anterior margins of the 

 mantle. (Hancock.)* 



The holes of the litliodomi often serve to shelter other animals 

 after the death of the rightful owners ; species of Modiola, Area, 

 Veneriqiis, and CoraUtophaga, both recent and fossil, have been 

 found in such situations, and mistaken for the real miners. f 



The boring shell-fish have been called '* stone-eaters " 

 {lithophagi) and "-wood-eaters" [xylophagi), and some of them 

 at least are obliged to swallow the material produced by 

 their operations, although they may derive no sustenance from 

 it. The ship-worm is often filled with pulpy, impalpable 

 sawdust, of the colour of the timber in which it worked. 

 (Hancock.) No shell-fish deepens or enlarges its burrow after 

 attaining the full growth usual to its species (p. 35). 



The bivalves live by filtering water through their gills 4 

 Whatever particles the current brings, whether organic or 

 inorganic, animal or vegetable, are collected on the surface 

 of the breathing-organ and conveyed to the mouth. In this 

 manner they help to remove the impurities of turbid water.§ 

 The mechanism by which this is eflPected may be most conve- 



* All attempts to detect the presence of an acid secretion have hitherto failed, as 

 might be expected ; for the hj-pothesis of an acid solvent supposes oulj" a verj- feeble 

 but continuous action, such as in nature always works out tlie greatest results in the 

 end. See Liebig's Organic Chemistry, and Dumas and Boussingault on the "Balance 

 of Organic Nature." Intimately connected with this question are several other 

 phenomena; tlie removal of portions of the interior of univalves, by the aniiTiiU 

 itself, as in the genera Conus, Auricula, and Nerita (Fig. 24, p. 32) ; the perforation of 

 shells by the tongues of the carnivorous gasteropods, and the formation of holes in 

 wood and limestone by limpets. Some facts in surgery also illustrate tliis subject, 

 (1) dead bone is removed when granulations grow into contact with it : (2) if a hole is 

 bored in a bone, and an ivory peg driven into it, and covered up, so much of the peg ;i3 

 is embedded in the bone will be removed. (Paget.) The *' absorption " of the fangs 

 of milk-teeth, preiious to shedding, is well known. In these cases the removal of the 

 bone earth is etfected without the development of an acid, or other distm-bance of the 

 neutral condition of the circulating fluid. 



t Fossil univalves (trochi) occupying the buiTows of a pholas, were di.«covered by 

 Mr. Bensted in the Kentish-rag of Maidstone. See Mantell's Medals of Creation. 

 M. Buvignier has found several species of Area fossilised in the burrows of 

 lit/iodoDH. 



X It seems scarcely neccssaiy to remark that the bivalves do not feed upon prey 

 caught between their valves. Microscopists are well aware that sediment taken from 

 the alimentary canal of bivalve shellfish contains the skeletons of animalcules and 

 minute vegetable organisms, whose geometrical forms are remarkably varied and 

 beautiful ; they have also been obtained (in greater abundance than ordinaiy) from 

 mud filling the interior of fossil oyster-shells. 



^ When placed in water coloured with indigo, they will in a short time render it 

 <]ear, by collecting the minute particles and condensing 'hem into a solid form. 



