The organization and replacement of the epidermis in acoelous turbellarians

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Abstract
Five species of acoelous turbellarians have been studied: Convoluta roscoffensis Graff, C. convoluta (Abildgaard), Aphanostoma diversicolor Oersted, Otocelis rubropunctata (Schmidt), and Avagina incola Leiper. A culture-method for C. roscoffensis is described. Experiments have been carried out with various preparative methods for electron microscopy, and a method that yields adequate results with acoel material is described. The epidermis is not syncytial, as has been maintained by some authors, but is composed of distinct, uninucleate cells. The cytoplasm of these has never been found to be continuous between cells, nor between the epidermis and the tissues that lie below it. The basal surfaces of the cells are intimately associated with the muscle layers and with other peripheral tissues. A brief account of the cytology of the epidermal cell is given. The fine structure of the cilia and their rootlets is described from observations on sectioned material and on isolated, ‘negatively stained’ specimens. The rootlets of adjacent cilia make contact with each other in such a way asto preserve a hexagonal arrangement of the cilia, and yet to allow some degree of dimensional flexibility. The organization of the ciliary array over the surface of the animal is described. The ciliary ‘streamlines’ radiate outwards in all directions from a region (occupied by the frontal gland) at the anterior tip of the animal, and converge on a point at the posterior tip. This pattern is interrupted only in small regions near the mouth and the genital openings. The directions of beat and of metachronal-wave propagation are uniformly related to each other, and to the structural pattern, at all points. Epidermal replacement has been studied experimentally in C. roscoffensis. Batches of animals were subjected to a surface-abrasive treatment, and epidermal-replacement cells were later identified in the parenchyma. The structure and situation of these is described, though their origin is not known. The findings are discussed in relation to the variety and possible functions of ciliary rootlets, and to current concepts of acoel organization.