The Significance of the Placophora for Molluscan Phylogeny(<Special Number>the 2nd International Chiton Symposium)

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  • 軟体動物系統発生における多板類の重要性(<特集号>第2回国際ヒザラガイシンポジウム)
  • The significance of the Placophora for mollusczm phylogeny
  • significance of the Placophora for mollusczm phylogeny

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Abstract

The organisation of the Placophora is compared with that of the Conchifera and of the aplacophoran Solenogastres and Caudofoveata. This analysis, with special emphasis on the threefold regionated alimentary tract and the excretory system, not only reveals a close relationship of the Placophora with the Tryblidia (part of paraphyletic *Monoplacophora*), but also recognises that the Placophora and Conchifera represent a monophyletic group of Testaria. Synorganisationally, other characters shared (as *Aculifera*) only with the aplacophoran molluscs represent plesiomorphies rather than synapomorphies. In contrast to some earlier assumptions, the Placophora cannot be regarded as being derived from the conchiferan level; rather, there is a well-defined organisational sequence of gradually additive characters (anagenesis) from the a-placophoran to the poly-placophoran and to the mono-placophoran (conchiferan) configuration. The organisation of the Placophora thus plays a key role in bridging the conservative aplacophoran and the derived conchiferan evolutionary levels within molluscs.

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