A light and electron microscopic study of the ciliated urn of Phascolosoma agassizii (sipunculida)

Summary The free swimming ciliated urn found in the coelomic fluid of Phascolosoma agassizii has been studied by electron microscopy. The urn is a multicellular structure composed of three cell types: (a) ciliated cells which possibly function in capturing cell debris and foreign particles; (b) cupola cells which are capable of phagocytozing latex particles; and (c) lobe cells which are capable of phagocytozing carbon particles. The lobes are separated from the ciliated cells by a semilunar area, with mucoprotein staining characteristics, containing fibrils which appear to be the structural support for the urn. Ciliated cells and lobes are attached to the semilunar area by hemidesmosomes.This work is based on a thesis submitted in partial fulfillment of the requirements for the degree Master of Arts in the Department of Biology, San Francisco State University, San Francisco, California, U.S.A.I wish to thank Dr. John C. Lee, formerly of the Department of Pathology, University of California Medical Center, San Francisco, for encouragement and use of his electron microscope facilities