92 
NATURAL HISTORY. [UPPER FLOOR. 
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 ex¬ 
tended, at its lower part, by the animals boring into the 
substance 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 ex¬ 
panded 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 MytiU, Pinnce, and Tridacnce^ or from a groove 
in the anterior and upper part of the edge of the right 
valve, as in the Pectmes, Aviculce^ and Mallei. 
The Anomice differ from the former, in being 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: others, as the Chamce jEtJierice, Spondyli, 
and Ostrece are attached by the outer surface of the shell 
to rocks, dec. These shells, or those which inhabit tubes, 
do not become attached until some time after they are ex¬ 
cluded 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 im¬ 
pression, which passes round the inner margin of the shell. 
But some shells, as the Cyclas, Cardium, and Loripes, with 
moderate syphons, have no such sinus, the muscular im¬ 
pression 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 cal¬ 
careous pearly secretions, either in the substance of their 
bodies or on the surface of their shells ; these secretions al¬ 
ways agree in colour with that of the inner surface of the 
shell to which the animal belongs. Thus those of the Pinna, 
