168 THE LIAS AMMONITES. 



SYSTEMATIC PART. 



THE STRUCTURE AND CLASSIFICATION OF THE CEPHALOPODA. 



The Cephalopoda form tlie highest class of the Mollusca, in which the head is 

 situated between the trunk and the arms, hence the name (/cE^aXjj, head, Troucfoot). The 

 existing forms are 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 muscular 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 musculo-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. 



