CONCHIFEUA. 243 



death of the rightful owners ; species of Modiola, Area, Venerupis, and Co- 

 ralliophaga, both recent and fossil, have been found in such situations, and 

 mistaken for the real miners.* 



The boring shellfish have been called " stone-eaters" {Hihophagi) 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 

 shellfish deepens or enlarges its burrow after attaining the full-growth usual 

 to its species (p. 43). 



The bivalves live by filtering water through their gills.f Whatever 

 particles the current brings, whether organic or inorganic, animal or vege- 

 table, 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. :j: 

 The mechanism by which this is effected may be most conveniently examined 

 in a bivalve with a closed mantle, like the great Mi/a (fig. 170), which lives in 

 the mud of tidal rivers, with only the ends of its long combined siphons 

 exposed at the surface. § The siphons can be extended twice the length of the 

 shell, or drawn completely within it ; they are separated, internally, by a 

 thick muscular wall. The branchial siphon {s) has its orifice surrounded by 

 a double fringe ; the exhalent siphon {s') has but a single row of tentacles ; 

 these organs are very sensitive, and if rudely touched the orifices close and 

 the siphon itself is rapidly withdrawn. When unmolested, a current flows 

 steadily into the orifice of the branchial siphon, whilst another current rises 

 up from the exhalent tube. There is no other opening in the mantle except 

 a small slit in front {p) through which the foot is protruded. The body 

 of the animal occupies the centre of the shell {b), and in front of it is the 

 mouth (o) furnished with an upper and a lower lip, which are prolonged on 

 each side into a pair of large membranous palpi {t). The gills {g) are placed 

 two on each side of the body, and are attached along tbeir upper, or dorsal 

 margins ; behind the body they are united to each other and to the siphonal 

 partition. Each gill is composed of two laminse, divided internally into a series 



* Fossil univalves [frochi) occupying the burrows of upholas, 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 fossilized in the burrows of 

 Itthodomi. 



+ 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 than ordinary) from 

 mud filling the interior of fossil oyster-shells. 



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

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



§ Alder and Hancock on the branchial currents of Fholas and Mya. An. Nat. 

 Hist. Nov. 1851. 



