136 
NATURAL HISTORY. ffiAST. ZOOL. 
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 
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 substance in which it is 
lodged. Some shells, as the Arcae> Nuculae , and Soleno - 
myce, attach themselves to rocks and stones, by a secre¬ 
tion which they emit from the expanded end of the foot: 
this secretion often hardens, and is calcareous. Other 
shells are attached by a beard, passing out either from the 
gape of the shell, as in the Mytili , Pinnae , and Tridacnce , 
or from a groove in the anterior and upper part of the 
edge of the right valve, as in the Pectines , Aviculae , and 
Mallei . The Anomiae 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 , Etheriae , Spondyli , and Ostreae , 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 remains of the young shells, which at first are 
not distorted, as they afterwards become, are often to be 
seen on the outside of the umbones of the parent shells. 
The animals of most of the larger species of these shells 
are used for food in various parts of the world. Many 
of them are liable to a disease which causes them to form 
calcareous pearly secretions, either in the substance of 
their bodies or on the surface of their shells; these secre¬ 
tions always agree in colour with that of the inner sur¬ 
face of the shell to which the animal belongs. Thus 
those of the Pinnae are pale brown and transparent; 
those of the Oyster are white and opaque; and those of 
the Muscles are either white or purple ; while those of 
the shells which have a pearly lustre, as the Aviculce , 
Uniones , and Anodons , partake of the same mild brilliancy. 
As the peculiar lustre of Pearls greatly depends on their 
more or less globular form, the Chinese have attempted, 
not very honestly, to make the pearly inside coat of 
some of the pond-muscles assume that shape, by placing 
