INTRODUCTION. 3 
give a general notion of the naked forms. The 
shelled species are familiar to us. The nervous 
system of the molluscous animals presents very 
marked peculiarities: in the back-boned and 
jointed animals, the principal mass of the nervous 
system, comprising the brain and spinal column, 
forms a continuous trunk throughout the length 
of the body, from which the nerves branch off at 
determinate points; whereas in the soft-bodied 
animals, the nervous system is composed of two 
or more pairs of brain-like masses scattered 
throughout the body, and united by cords of 
nerve-substance, which also send off nerves to 
the several parts of the body. 
The sub-kingdom Mollusca admits of a very 
ready division into minor groups or classes. 
Thus we have cuttle-fishes, sea snails, land 
snails, bivalves, &c. In the language of the 
naturalist, the cuttle-fishes are the Cephalopoda, 
or head-footed mollusks; the sea snails are the 
Gasteropoda, or belly-footed mollusks; the land 
snails somewhat resemble the sea snails, but 
breathe air instead of water, and are hence 
termed Pulmonifera; the bivalves, as the mussel 
and oyster, are the Conchifera. There are four 
other classes of the Mollusca, but which, to- 
gether with the cuttle-fishes, do not come into 
the scope of the present subject, because they 
are all marine; so that all our land and fresh- 
B 2 
