108 
NATURAL HISTORY. [NEW BUILDING. 
to separate the lower edges of the valves from one another, 
without any exertion to the animal, while it is waiting with 
its mantle leaves open in quest of its prey, or walking 
about to find a place more agreeable to its habits or where 
food may be more abundantly procured. That the carti¬ 
lage is distinct from the ligament, is easily proved by the 
fact of its being often separated from it and placed in a 
triangular pit on the hinge-margin of the valves, when 
the shell is said to have an internal cartilage, as in Pecten , 
Mactra , &c. In these the fibres of the cartilage are directly 
pressed by the surface of the valves, as it is again pressed 
by the edge of the valves against the inner surface of the 
ligament in those shells which, have the cartilage situated 
immediately within that body. In some few, as the Pid- 
docks (Pholas), there is no cartilage, its place being 
supplied by muscles, which are attached to the posterior 
edge of the valves, and are covered by a thin skin in¬ 
stead of a ligament, in which shelly plates are usually im¬ 
bedded. The ligament and cartilage are increased in 
size as the animal grows by the addition of new particles 
to their hinder edge, which are deposited by the fold of 
the mantle, situated immediately within them, between the 
edges of the valves, the ligament being always first en¬ 
larged so as to form a protection to the cartilage which is 
afterwards produced below it. 
The Conchifera 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, or the hinder end of the body, 
near the surface, and their mouths downwards. Others, 
as the Petricolce , Lithodomi , and Pholades , form for 
themselves holes in calcareous rocks or old shells, in 
which they constantly remain during the whole of their 
lives. Some few line these holes with a calcareous 
secretion, as the Gastrochceiuz and Teredines; Clavagella 
and Aspergillum 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 exter¬ 
nally. Those animals which fix the valves to their tubes, 
have the ends thereof pierced with holes for the passage 
