Degenerating epidermal cells in Xenoturbella bocki (phylum uncertain), Nemertodermatida and Acoela (Platyhelminthes)

Species of the Nemertodermatida. and Acoela have a mode of withdrawing and resorbing worn ciliated epidermal cells, through the epidermis and into the gastrodermal tissue. The ultrastructure of these degenerating epidermal cells has been described from very few species, especially concerning the Nemertodermatida. New data are presented here from Nemertoderma bathycola. Studies of the body wall of the enigmatic Xenoturbella bocki revealed darkened, shrunken cells with epidermal-type cilia enclosed in a vacuole. These cells were found in basal parts of the epidermis and within gastrodermal cells. The cells, or remains of them, were more dissolved in structure the further into the body they were observed. The process of resorbing ciliated epidermal cells in X bocki is essentially similar to that found in the Acoela and Nemertodermatida, thus supporting the hypothesis of a close relationship.