108 
ROOM XIII. 0 f its attachment to the rock: others, as the Chamce 
Nat. Hist. JEtherice, Spondyli , and Ostrece are attached by the outer 
surface of the shell to rocks, &c. These shells, or those 
which live in tubes, do not become attached until some 
time after they are excluded from the egg: the young 
shells, which at first are not distorted, are often to be 
seen on the outside of the umbones of the parent shells. 
The lobes of the mantle of these animals are often 
united behind, and extended into longer or shorter 
syphons, through which the water passes to the gills. 
Most of the shells, which have these tubes long and 
free, are marked with a deep sinus in the hinder part of 
the muscular impression, which passes round the inner 
margin of the shell. But some shells, as the Cyclas , Car - 
dium and Lorijpes , with moderate syphons, have no such 
inflection, the muscular impression continuing parallel 
to the edge, as in those animals which have the lobes 
of their mantle quite separate, except on the dorsal 
margin. 
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 secretions, either in the sub¬ 
stance of their bodies, or on the surface of their shells; 
these secretions always agree in colour with that of 
the inner surface of the shell to which the animal 
belongs. Thus those of the Pinna, 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 Avicula , 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, for no very honest purpose, to make the 
pearly inside coat of some of the pond-muscles assume 
that shape, by placing hemispherical pieces of mother 
of pearl, between the animal and the shell, which it 
eventually covers with a pearly coat, (see case 83). 
In 
