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 called _Brachiopoda_, a class which we may, at least provisionally, consider as belonging to the mollusca. These _Brachiopods_ are also called "Lamp-shells," from a certain resemblance which many of them show to the form of a classical lamp. They are interesting, because in very ancient times they seem to have held that place in the world's animal population which is now held by the Lamellibranchs, by which, as they died out, they have been gradually replaced till but comparatively few forms survive. Some of these, however, are of great antiquity, and one of them, _Lingula_, is, though still living, one of the most ancient of all known animals.