NATURAL HISTORY. 
73 
GALLERY.] 
with respect to each other, while the use of the cartilage is 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 cartilage 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 di¬ 
rectly 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 Piddocks ( 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 instead of a ligament, in which 
shelly plates are usually imbedded. The ligament and cartilage are in¬ 
creased 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, si¬ 
tuated immediately within them, between the edges of the valves, the 
ligament being always first enlarged 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 r oot, forming for themselves holes in the sand or mud on 
the sea-c^ast, in which they rest with their syphons, or the hinder end 
of the body, near the surface, and their mouths downwards. Others, 
as the Petricolee , 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 Gaslrochcence and Teredines; 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 for the 
passage of filaments, 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 sub¬ 
stance in which it is lodged. Some shells, as the Arcce , Nuculce , and 
Solenomyce, 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 beard, pass¬ 
ing out either from the gape of the shell, as in the Mytili , Pinnce, and 
Tridacnce , 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 are fixed by a muscle passing out of a deep notch in the under 
valve, which secretes a hard disk at the places of its attachment to the 
rock: and others, as the Chamce, Etheriee , Spondyli , and Ostrece , are 
attached by the outer surface of their shell to rocks, &c. These shells, 
and those which inhabit tubes, do not become attached until some time 
after they are excluded from the egg: the nucleus, or shell, of the 
young animals, which at first are not distorted, as they afterwards be- 
