NATURAL HISTORY. 
MOLLUSCA. 
THe four Classes of animals which have been 
considered in the preceding volumes of this series 
we have seen to have one character in common; 
viz. the possession of a bony framework within the 
body, of which a jointed spine is the most essential 
element. This character, which unites those four 
Classes into one great group, and gives to that 
group the name VERTEBRATA, by which it is dis- 
tinguished among naturalists, we have seen, how- 
ever, by slow degrees, deteriorated, if I may use 
such an expression, from bone to cartilage, and 
gradually diminished in its development, until, in 
the lowest of the Fishes, it can scarcely be recog- 
nised at all. 
I come now to treat of animals in which the 
bony skeleton no longer exists. The conditions of 
their existence do not require such a scaffolding on 
which to build the constituent muscles: many are 
habitually immersed in water, a fluid the density of 
which supports their soft bodies; their motions 
generally lack the precision, energy and variety of 
B 
