14 MOLLUSCA. 
buried, the animal is still enabled to communicate 
with the water by its respiratory siphon.” * 
Beyond the rudimentary strip of cartilage that 
in some of the Cephalopoda represents the vanish- 
ing spine of the VERTEBRATA, the MoLLuscA have 
no internal skeleton. But in the great majority of 
cases, the soft parts are protected and supported by 
what we may call an external skeleton, of the sub- 
stance familiar to us as shell. Lime is the es- 
sential element of this substance, as it is also of 
bone: but shell is a carbonate of lime, while the 
earthy part of bone 1s a phosphate. 
When we consider the beauty and variety that 
are presented by shells, the important part they 
play in the economy and habits of the animals, 
and the use that is made of them in systematic 
arrangement, it becomes a question of high interest 
to inquire in what manner they are formed. 
“The shells themselves are absolutely deprived 
of vitality, permeated by no vessels, and as inca- 
pable of expansion by any internal power as the 
rocks to which they are not uncommonly attached; 
so that the young naturalist is necessarily at a loss 
to conceive either the mode of their formation, or 
the origin of all the gaudy tints and external de- 
corations that render them the ornaments of our 
cabinets. 
“The simple apparatus by means of which shells 
are constructed, is the external membranous layer 
that invests the body of the mollusk,—the mantle, 
as it has been termed ; and, whatever the form of 
the shell, it owes its origin entirely to this delicate 
organ. : : , 
“Tt is the circumference, or thickened margin of 
* Introduction to Conchology, p. 173. 
