le many are altogether naked. In some kinds the soft
body is drawn out into a number of tufted processes, as in Doris and
Eolis, and sometimes the body is almost worm-like, as in _Phylliroe_, or
provided with a pair of ring-like lateral processes and a rudimentary
shell, as in the sea-hare _Aplysia_.

Next above the Gasteropods comes a group of animals forming the class
_Pteropoda_. These pteropods are small, active, oceanic,
surface-swimming creatures, many of which live in delicate glass-like
shells, and some of which form a large part of the food of the whalebone
whale. They flit through the water by the aid of lateral processes which
much resemble those before-mentioned as existing in the sea-hare. Allied
to these pteropods is a curious little animal, the shell of which
resembles a miniature elephant's tooth and which is named _Dentalium_.

Highest of all the mollusca stand the cuttle-fishes, forming (with the
_Nautilus_ and many extinct animals, such as ammonites and their allies)
the great class _Cephalopoda_. The Cephalopoda, such as the cuttle-fish
(_Sepia_) and the Poulp (_Octopus_), have now become familiar objects
through our aquaria, where their very eccentric forms and remarkable
movements naturally attract attention. To this group also belongs
_Spirula_, the coiled and chambered shell of which is found so
abundantly, but its soft tenant so very rarely. To it also belongs the
extinct Belemnite, which was provided with a dense, conical internal
shell, specimens of which found in rocks were at one time taken for
thunderbolts. Of a lower grade of organization is the _Nautilus_, sole
existing representative of a great group of Cephalopoda (including the
ammonites and other forms) which has, with the above exception, long
become entirely extinct.

The oyster is an animal which belongs to a much lower class of
mollusca--namely, to the class called _Lamellibranchiata_, from the
plate-like (or lamellar) structure of the gill. To that class also
belongs the scallop (_Pecten_), the mussel (_Magilus_), the fresh-water
mussel (_Anodon_), the razor-shell (_Solen_), the cockle (_Cardium_),
species with a long fleshy tube such as _Mya_, stone-perforating shells
such as _Pholas_, and the well-known wood-boring "ship-worm"
(_Teredo_)--which is no "worm" at all--with a multitude of other forms.

Certain other animals (which, like the Lamellibranchs, all have a shell
divided into two valves) form another still lower class