]
Lastly, the Lower Silurian Rocks have yielded a vast number of
chambered shells, referable to animals which belong to the same
great division as the Cuttle-fishes (the _Cephalopoda_), and
of which the Pearly Nautilus is the only living representative
at the present day. In this group of _Cephalopods_ the animal
possesses a well-developed external shell, which is divided into
chambers by shelly partitions ("septa"). The animal lives in
the last-formed and largest chamber of the shell, to which it
is organically connected by muscular attachments. The head is
furnished with long muscular processes or "arms," and can be
protruded from the mouth of the shell at will, or again withdrawn
within it. We learn, also, from the Pearly Nautilus, that these
animals must have possessed two pairs of breathing organs or
"gills;" hence all these forms are grouped together under the
name of the "Tetrabranchiate" Cephalopods (Gr. _tetra_, four;
_bragchia_, gill). On the other hand, the ordinary Cuttle-fishes
and Calamaries either possess an internal skeleton, or if they
have an external shell, it is not chambered; their "arms" are
furnished with powerful organs of adhesion in the form of suckers;
and they possess only a single pair of gills. For this last reason
they are termed the "Dibranchiate" Cephalopods (Gr. _dis_, twice;
_bragchia_, gill).
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