107 
some few, as the Piddocks ( Pholas ), there is no carti- ROOM XIII. 
lage, its place being supplied by muscles, which are n at . Hist. 
attached to the posterior edge of the valves, which are 
covered by a thin skin, instead of a ligament, in which 
shelly plates are usually imbedded. The animals of 
bivalve shells are in general free, and walk about by 
means of their compressed foot, forming for themselves 
holes in the sand or mud on the sea-coast, in which 
they rest with their syphons near the surface, and 
their mouths downwards. Others, as the Petricolce , 
Lithodomi , and PJiolades , form for themselves holes 
in calcareous rocks or old shells, in which they con¬ 
stantly remain during the whole of their lives. Some 
few line these holes with a calcareous secretion, as the 
GastrocJuznce and Teredines. The Clavagella and Asper¬ 
gillum form testaceous tubes, to which the former fixes 
one of its valves, leaving the other free to move at 
the will of the animal, while the latter fixes them 
both, so that the valves appear to form a part of 
the tube, their apices only being visible externally. 
Those animals which fix the valves to their tubes, 
have the ends thereof pierced with holes, and they 
only appear to increase it, at its upper or exposed 
hinder edge; while in those in which the valves are 
free, the case is extended, at its lower part, by the 
animals boring into the substance in which it is lodged. 
Some shells, as the Arcce , Nuculee , and Solenomyoe , 
attach themselves to rocks and stones, by a secretion 
which they emit from the expanded end of the foot: 
this secretion often hardens, and is calcareous. Other 
shells are attached by a Byssus, which arises from a 
sheath at the base of the front part of the foot, and 
is projected either from the gape of the shell, as in the 
Mytili , Pinnce , and Tridacuce , or from a groove in 
the anterior and upper part of the edge of the right 
valve, as in the Pectines , Aviculce , and Mallei . 
The anomicE differ from the former, in being fixed 
J o 
by a muscle passing out of a deep nick in the un¬ 
der valve, which secretes a hard disk at the places 
of 
