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American Zoologist 1994 34(4):523-532; doi:10.1093/icb/34.4.523
© 1994 by The Society for Integrative and Comparative Biology
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Epithelial Origin of Mesodermal Structures in Arrowworms (Phylum Chaetognatha)1

GEORGE L. SHINN
Division of Science, Northeast Missouri State University Kirksville, Missouri 63501

SYNOPSIS. The classical theory that coelomates evolved from pseudocoelomate and acoelomate ancestors implies that mesodermally derived tissues such as body wall muscles, somatic reproductive tissues, and connective tissues are homologous throughout the Triploblastica and distinct from the epithelial coelomic lining. Ultrastructural studies reveal the mesoderm of hatchling chaetognaths to consist entirely of an epithelial coelomic lining that includes the longitudinal body wall muscles. Comparisons of hatchling and adult mesodermal organization are used to infer that all adult mesodermally derived tissues and organs form by localized elaboration of the hatchling coelomic lining. These findings provide no basis for inferring that any adult mesodermally derived structures evolved independently of or prior to the coelomic epithelium. The presence in hatchlings of myoepithelial body wall muscles supports the theory that coelomates evolved by elaboration of an epithelial coelomic lining from the myoepithelial gastrodermis of diploblastic ancestors.


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