240 LIFE, IN ITS INTERMEDIATE FORMS. 
cept that there is an opening in the side for the protru- 
sion of a moveable fleshy organ called the foot ; but ex- 
ternally there is this peculiarity, that, in the substance of 
the coat, there are formed two valves or convex plates of 
stony substance (shelf), which arc united along one side 
by a hinge, and enclose the soft parts, as the leaves of a 
book are contained within the covers. 
This, then, is a bivalve shcll-y&A, as it is commonly 
(thougli incorrectly) named, and it is the representative 
of an extensive Glass of animals, found all over the world, 
in fresh and salt waters, and designated Conchifera, or 
u Shell-bearers. there is, it is true, very great diversity 
in the details of form and structure that we find in this 
immenso assemblage of animals ; but all these variations 
can be easily traced by insensible gradation to this primal 
form, and thence to those lower types which we have 
already described. Sometimes the orifices for receiving 
and discharging water are prolonged into two distinct 
tubes with fringed extremities, as in that lovely and deli- 
cate shell that inhabits our sandy beaches, called, from 
its diverging rays of pink and yellow, the Setting Sun 
(Psammolia vesper tina), where the two tubes, when fully- 
extended, are twico the length of the shell. At other 
times we find the tubes again reduced to simple openings, 
and one of these forming a mere slit, scarcely to be dis- 
tinguished from the common opening of the coat or mantle, 
as in the Mussels ( Mytilidce ). This separation of the 
mantle, again, occurs in various degrees, from its condi- 
tion as a mere orifice for the protrusion of the foot, to 
that of the Oysters (Ostreadce), where it is open all round, 
a fleshy counterpart of the shelly valves, bordered by a 
