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Animal EvolutionInterrelationships of the Living Phyla$
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Claus Nielsen

Print publication date: 2011

Print ISBN-13: 9780199606023

Published to Oxford Scholarship Online: December 2013

DOI: 10.1093/acprof:oso/9780199606023.001.0001

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Phylum Mollusca

Phylum Mollusca

Chapter:
(p.140) 27 Phylum Mollusca
Source:
Animal Evolution
Author(s):

Claus Nielsen

Publisher:
Oxford University Press
DOI:10.1093/acprof:oso/9780199606023.003.0027

Mollusca is comprised of approximately 200,000 described living species, making it one of the largest animal phyla, and consists of eight living classes that are predominantly marine, with gastropods and bivalves the only ones to have spread to freshwater. Only the gastropods have entered the terrestrial habitat, and some families of snails and mussels are commensalistic. The living molluscs are a morphologically very well-defined, monophyletic group, and show a number of characters that can be interpreted as ancestral of the phylum. The most recent molluscan ancestor could have had three characters: mantle, foot, and radula. Aplacophora and Testaria are two evolutionary lines that can be traced from the ancestral mollusc with a ciliated foot and a mantle with a mucopolysaccharide lining with calcareous spicules. On the basis of the teloblastic growth of the 4d mesoderm in both bivalves and gastropods, the molluscs probably form a monophyletic group with sipunculans and annelids.

Keywords:   molluscs, Mollusca, gastropods, bivalves, snails, mussels, mantle, radula, Aplacophora, Testaria

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