CONCHIFERA. 395 



its crj'pt by a hyssus. These shell fish, have been supposed to 

 clissolye the rock by chemical means (Deshayes), or else to 

 wear it away with the thickened anterior margins of the 

 mantle. (Hancock.)* 



The holes of the litJwdomi often serve to shelter other animals 

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

 Venerupis, and Coralliophaga, both recent and fossil, have been 

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



The boring shell-fish haye been called " stone-eaters " 

 {litliophagi) and *' wood-eaters" [xyJophagi)^ 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 gills4 

 T\Tiatevcr 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 eSected may be most conve- 



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

 might be expected ; for the hypothesis of an acid solvent supposes only a very feeble 

 but continuous action, such as in nature always -works out the gi-eatest results in the 

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

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

 phenomena; the removal of portions of the interior of univalves, by the animal 

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

 sliells by the tongues of the carnivorous gasteropods, and tlie foimation of holes in 

 wood and limestone by limpets. Some facts in sm-gery also illustrate this 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 pe^ as 

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

 of milk-teeth, previous to shedding, is well known. In tliese cases the removal of the 

 bone earth is effected without the development of an acid, or other disturbance of the 

 neutral condition of the circulating fluid. 



t Fossil univalves (frocAi) occupying the burrows of a pholas, were discovered 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 cf 

 lithodomi. 



X It seems scarcely necessary 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 tlian ordinarj-) from 

 mud filling the interior cf fcesil oyster-shells. 



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

 clear, by collecting the minute particles and condensing tJiem into a solid form. 



