FINE-STRUCTURE OF DICYEMID MESOZOANS, WITH SPECIAL REFERENCE TO CELL-JUNCTIONS

The fine structure of the dicyemid mesozoan, Dicyema acuticephalum, fr om Octopus vulgaris, was studied with special attention to intercellul ar junctional complexes between various kinds of cells. Two types of i ntercellular junction, namely, adherens junctions and gap junctions, w ere found in both vermiform stages and in infusoriform embryos. Adhere ns junctions were classified into two types. Zonulae adherentes-like j unctions were observed between adjacent peripheral cells at vermiform stages, between adjacent external cells of infusoriform embryos, and b etween members of groups of internal cells that covered the urn in inf usoriform embryos. Maculae adherentes-like junctions were seen between a peripheral eel and an axial cell at vermiform stages. In infusorifo rm embryos, these junctions were observed between various types of cel ls, excluding urn cells. Gap junctions were found between adjacent per ipheral, cells at vermiform stages, whereas in infusoriform embryos th ese junctions were located between various types of cells excluding ur n cells. Dicyemids might be the most primitive multicellular animals t o possess these basic types of cell junctions. Ciliary rootlet systems at vermiform stages and in infusoriform embryos were unique in struct ure compared with those of other primitive multicellular animals. (C) 1997 Wiley-Liss, Inc.