Degenerating epidermal bodies (“pulsatile bodies”) inMeara stichopi (Plathelminthes, Nemertodermatida)

Abstract

The original description of the nemertodermatid flatworm,Meara stichopi, reported the occurrence of distinct “restitution cells” in the epidermis. An ultrastructural investigation of these objects was undertaken. 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 regeneration. They are old epidermal cells which are with-drawn from the epidermis to the digestive tissue, evidently for resorption. A peculiarity of the degenerating epidermal cells ofM. stichopi is that the cilia are most often detached before the cell is withdrawn from the epidermal surface, resulting in an inability to pulsate.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 synapomorphic character state for the Nemertodermatida and the Acoela.