ourselves belong. Nevertheless, since the members of the invertebrata
sub-kingdoms are, speaking generally, much less known and familiar than
are vertebrate animals, and as the structural differences between them
cannot be pointed out till an initial acquaintance has been made with
comparative anatomy, for these reasons we may treat the various animal
sub-kingdoms which have yet to be noticed at much less length than we
have treated the vertebrata. The details of their peculiarities and the
various degrees of significance and interest which they present will
begin to appear when we proceed to treat of "The Forms of Animals."

The last class of vertebrates is, as we have seen, constituted by the
fishes, which are fishes properly so called. But there are many animals
which are familiarly and improperly spoken of as "Fishes," but which are
even more below true fishes than whales and porpoises are above them.
Thus, we hear of cuttle-fishes, and a variety of creatures are spoken of
as "shell-fish," which are not in the least related to true fishes.
Indeed, the many so-called "shell-fish" are not even nearly related one
to another. Thus, the oyster and the lobster are both commonly thus
named, but they belong respectively to two altogether distinct
sub-kingdoms of the world of animals.

The oyster is an animal which belongs to a vast assemblage of species,
with much variety of form and structure, which, on account of their soft
bodies (whether or not enclosed in shells), are called MOLLUSCA or
"Mollusks." This assemblage ranks as a sub-kingdom and contains within
it at least four subordinate great groups, or "classes." All snails and
whelks, with their allies, and also all cuttle-fishes, belong to the
sub-kingdom of "soft animals."

Amongst the most familiar of mollusks is the common snail, which may
serve as a type of the "class" of mollusks to which it belongs--the
class _Gasteropoda_. The snail, with the slug, are representatives of
land-forms of mollusca, but the bulk of the class and of the whole
sub-kingdom are aquatic animals, such as the whelk (_Buccinum_),
periwinkle (_Littorina_), limpet (_Patella_), &c. The Gasteropods
generally possess spirally coiled shells (like the cowry or whelk), but
some kinds have their shells in the form of simple cones--like a
Chinaman's cap--as, _e.g._, the limpet. There are a few Gasteropods in
which the shell consists of a series of similar segments as is the case
with _Chiton_, whi