A light and electron microscopic study of the ciliated urn of Phasolosoma agassizii (Sipunculida)

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.