Cephalopoda

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(Class Cephalopoda: modifications)
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==Class Cephalopoda==  
==Class Cephalopoda==  
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===Basic Divisions===
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====Basic Divisions====
*Phyllum Mollusca
*Phyllum Mollusca
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Most classification divide the Cephalopoda in the three subclasses as shown. Nautiloids are also known as Palcephalopoda, a term which is applied to most but not all members of the subclass.  Ammonoids and Coleoids and a few nautiloides come under the term Neocephalopoda. Both terms are based on the morphology and biology of living cephalopods, Palcephalopoda for the presumed primitive ''Nautilus'' and ''Allonautilus'', representing the nautilids and Neocephalopoda for the derived and presumed advanced coleoids.                   
Most classification divide the Cephalopoda in the three subclasses as shown. Nautiloids are also known as Palcephalopoda, a term which is applied to most but not all members of the subclass.  Ammonoids and Coleoids and a few nautiloides come under the term Neocephalopoda. Both terms are based on the morphology and biology of living cephalopods, Palcephalopoda for the presumed primitive ''Nautilus'' and ''Allonautilus'', representing the nautilids and Neocephalopoda for the derived and presumed advanced coleoids.                   
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===Systematics===
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====Systematics====
The class Cephalopoda has been traditionally divided into three subclasses on the basis  of shell structure, or two subclasses on the basis of gills and other soft parts. Both shown here:
The class Cephalopoda has been traditionally divided into three subclasses on the basis  of shell structure, or two subclasses on the basis of gills and other soft parts. Both shown here:
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Shell based classification, used exclusively in paleontology, with three basic subclasses.
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>Shell based classification, used exclusively in paleontology, with three basic subclasses.
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'''Nautiloidea''' -- includes primitive forms, all have an external shell and a retrosiphonate siphuncle. Modern representatives, ''Nautilus'' and  ''Allonautilus'' are tetrabranchiate.  
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<u>Nautiloidea</u> -- includes primitive forms, all have an external shell and a retrosiphonate siphuncle; represented by modern, tetrabranchiate ''Nautilus'' and  ''Allonautilus''.  
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'''Ammonoidea''' -- evolved forms with an external shell and typically a prosiphonate siphuncle. Derived from the Nautilioidea but closely related biologically to the dibranchiate coleoids . Became extinct at the end of the Cretaceous.
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<u>Ammonoidea</u> -- evolved forms with an external shell and typically a prosiphonate siphuncle; derived from the Nautilioidea but closely related biologically to the dibranchiate coleoids. Became extinct at the end of the Cretaceous.
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'''Coleoidea''' -- shell-less forms and those with an internal vestigial shell represented by octopods and dibranchiate squid, cuttlefish and ''Spirula'' that live today. Includes the extinct belemnites.
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<u>Coleoidea</u> -- shell-less forms and those with an internal vestigial shell represented by living octopods, cuttlefish, squid,and ''Spirula'', all dibranchite. Includes belemnites and other extinct forms.
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Older, anatomical (gill) based classification.
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>>Anatomical (gill) based classification, used in cephalopod biology.
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'''Tetrabranchiata'''-- forms with four gills (2 pairs), represented by ''Nautilus'', thought to be primitive. May be equivalent to the Nautiloidea.
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<u>Tetrabranchiata</u>-- forms with four gills (2 pairs), represented by ''Nautilus'', thought to be primitive. The Palcephalopoda.
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'''Dibranchiata'''-- forms with two gills, considered advanced. Equivalent to the Coleoidea, possibly the Ammonoidea and may be even some Nautilodea
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<u>Dibranchiata</u>-- forms with two gills, considered advanced. Equivalent to the Coleoidea, possibly the Ammonoidea and may be even some Nautilodea
The trouble with the anatomical scheme is that it is impossible to know from shell structure alone alone whether extinct forms had two or four gills, or anything really much about their soft body anatomy. From this it is apparent that the two-fold anatomical classification is inadequate except in modern forms.   
The trouble with the anatomical scheme is that it is impossible to know from shell structure alone alone whether extinct forms had two or four gills, or anything really much about their soft body anatomy. From this it is apparent that the two-fold anatomical classification is inadequate except in modern forms.   

Revision as of 13:30, 8 December 2011

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