168 
THE LIAS AMMONITES. 
SYSTEMATIC PART. 
THE STRUCTURE AND CLASSIFICATION OF THE CEPHALOPODA. 
The Cephalopoda form the highest class of the Mollusca, in which the head is 
situated between the trunk and the arms, hence the name (/c£^aA?j, head, ttoGc, foot). The 
existing forms ^ire nearly all naked animals without shells, whilst the extinct groups were 
with a few exceptions the inhabitants of polythalamous shells, fashioned after the structure 
of the many-chambered shell possessed by the Pearly Nautilus. 
The trunk or body is symmetrical, thick, and soft, and has a spherical, oblong, 
elongated, or cylindrical form, enclosed in a fleshy inuscular sheath or mantle, which 
envelopes all the viscera, furnishes a pair of lateral fins or appendages for locomotion, and 
in the naked species lodges in its substance the rudiments of an internal skeleton. 
The head, more or less distinct from the trunk by a contraction or neck, is large and 
round ; it contains the organs of sense, mastication, and deglutition, and gives off from 
its anterior circumference a number of long fleshy processes or arms, which are either 
short hollow sheaths with retractile tentacles, as in the Pearly Nautilus, forming the 
order Tentaculifera (fig. 24), or form eight or ten solid arms, more or less elongated 
and flexible in all directions, supporting on their internal surface numerous suckers, by 
which the arms become powerful instruments for adhesion, prehension, and locomotion, 
hence the name Acetabulifera for this order (fig. 14). 
The mouth, situated at the bottom of a conical cavity formed at the base of the 
arms, is provided with two horny or calcareous mandibles, resembling in form those 
of a parrot, and like them playing vertically upon each other, and enclosing a large fleshy 
tongue, partly covered with short, recurved, horny spines. 
The two eyes are very large and placed on each side of the head ; they are comparable 
in their structure with those of the Vertebrata ; and are sessile in the Acetabulifera, 
pedunculated in the Tentaculifera. 
A fleshy tube, the infundibulum or locomotive pipe, forms the passage through which 
the respiratory currents of water to and from the branchial chamber, and the excrement 
from the intestine, are discharged ; it is a nuisculo-mucous organ situated at the anterior 
part of the neck, like an inverted funnel, with a pipe projecting from the visceral cavity 
and directed forwards and outwards. 
