Zoomorphology (1996) 116:1-5 9 Springer-Verlag 1996 Kennet Lundin 9 Jan Hendelberg Degenerating epidermal bodies ("pulsatile bodies") in Meara stichopi (Plathelminthes, Nemertodermatida) Accepted: 10 October 1995 Abstract The original description of the nemerto- dermatid flatworm, Meara stichopi, reported the occur- rence of distinct "restitution cells" in the epidermis. An ultrastructural investigation of these objects was under- taken. We found cell bodies with an ultrastructure similar to "pulsatile bodies" reported from Acoela. The cells are clearly the result of a degeneration process, not of regen- eration. They are old epidermal cells which are with- drawn from the epidermis to the digestive tissue, evi- dently for resorption. A peculiarity of the degenerating epidermal cells of M. stichopi is that the cilia are most often detached before the cell is withdrawn from the epi- dermal surface, resulting in an inability to pulsate. Thus, we suggest that the term "pulsatile body" is replaced by the more inclusive and informative term "degenerating epidermal body". The results support the hypothesis put forward by Ehlers that these bodies represent a synapo- morphic character state for the Nemertodermatida and the Acoela. A. Introduction "Pulsatile bodies" in the Acoela (Plathelminthes) have been reported from studies at the light microscopical level for a long time, but the nature of these structures has been debated. The name refers to the motion of a tuft of cilia enclosed in an intracellular vacuole. Al- though first regarded as possible endoparasites (Geddes 1879) or flame cells (Delage 1886), many authors con- sidered the "pulsatile bodies" of the Acoela as epider- mal replacement cells (for references, see Ehlers 1992b). Some authors did, however, conclude that the "pulsatile bodies" are degenerating epidermal cells (Be- klemischev 1915; Mamkaev 1965). Dorey (1965) pub- lished the first ultrastructural study of the "pulsatile K. Lundin ( ~ ) 9 J. Hendelberg Department of Zoology, University of G6teborg, Medicinareg. 18, S-413 90 G6teborg, Sweden bodies" of a species of the Acoela. He regarded them as replacement cells but discussed the possibility that they could be degenerating epidermal cells, since he ob- served several "pulsatile bodies" undergoing degenera- tive changes. Tyler (1984) considered the "pulsatile bod- ies" to be degenerating epidermal cells because only ful- ly differentiated cells had been observed in the process of being digested. Also, the ciliary substructure in the "pulsatile bodies" indicated resorption rather than cilio- genesis. Tyler et al. (1989) showed that "pulsatile bod- ies" appear at epidermal wounds in Convoluta pulchra Smith, 1991, and that they are damaged cells being re- sorbed. Ehlers (1992b), studying Anaperus tvaerminnensis (Luther, 1912) demonstrated that the "pulsatile bodies" are degenerating epidermal cells which are withdrawn from the epidermis and enclosed in vacuoles of the central digestive system. Hence, the question of the nature of the "pulsatile bodies" in the Acoela is undoubtly settled. "Pulsatile bodies" have been reported from a species of Nemertoderma by Smith et al. (1986), but no micro- graphs of "pulsatile bodies" of the Nemertodermatida have been published to date. In an earlier paper (Lundin and Hendelberg 1995), we described ordinary epidermal cells of Meara stichopi Westblad, 1949 (Nemertodermat- ida). In this paper, we report on a study of the ultrastruc- ture of the putative "pulsatile bodies" in M. stichopi and discuss the functional and phylogenetic significance of our observations. B. Materials and methods Specimens of Parastichopus tremulus (Gunnems, 1767) were col- lected in September 1994 by dredging at a depth of 120-130 m in the Fanafjord west of the marine biological station at Espegrend, Bergen, on the west coast of Norway. Several specimens of Meara stichopi were found in the intestine of these holothurians. The M. stichopi specimens were fixed in buffered 3% glutaraldehyde and postfixed in 1% osmium tetroxide; for details of fixation and TEM preparations, see Lundin and Hendelberg (1995). The sections were examined in a Zeiss CEM 902 transmission electron micro- scope.