24 MOLLUSCA. 
those which belong to Vertebrate animals; and 
where this is not the case, as in the Articulate 
Classes, the skeleton which affords attachment to 
the muscles, is not internal, but invests the body, 
while its substance differs essentially from bone in 
its chemical composition and its structure. 
An immense assemblage of living creatures are 
included in this category; creatures differing widely 
from each other in the most important character- 
istics, so that they cannot be grouped together. 
The term INVERTEBRATA, by which they are some- ° 
times designated, indicates indeed only a negative 
character, and we shall be greatly mistaken if we 
suppose (misled by such a term) that the animals 
which have a skeleton, and those which are des- 
titute of one, constitute two primary divisions ot 
living beings, of equal or co-ordinate importance. 
Several divisions of INVERTEBRATE animals do, 
in fact, exist, each one of which is equal in rank to 
the VERTEBRATA. One of these will form the sub- 
ject of the present volume, commonly known by 
the name of MoLLusca; a term invented by the 
illustrious Cuvier, from the word mollis (soft), and 
evidently suggested by the softness of their bone- 
less bodies. The appellation can scarcely be con- 
sidered happy, for the character so indicated is 
very trivial, and is shared by other animals of 
totally different structure : — objectionable, how- 
ever, as it is, it has been generally adopted, and 
I shall not hesitate to make use of it. 
As the great Vertebrate Division includes the 
four distinct Classes of Beasts, Birds, Reptiles and 
Fishes, so does the great Division of MoLLUscA 
contain six Classes, distinguished by characters 
which I shall presently enumerate. J must, how- 
