Fine-structural observations on the central parenchyma in Convoluta sp.

Abstract

Although the turbellarian order Acoela occupies a significant position in many theories on the origin of the Metazoa, detailed ultrastructural observations on members of this group are few in number. The acoelan central parenchyma has been recently described as at least partly syncytial on the basis of electron-microscopical data. These data, however, are insufficient for a complete understanding of the anatomical construction of the acoelan central parenchyma. In the present paper, a description of the micro-anatomy of the central parenchyma in juveniles of Convoluta sp. is presented. This region of the body consists of a central syncytium and certain specialized ‘wrapping cells’ which are associated with the syncytium. The wrapping cells are tentatively placed in three classes based on their positions and morphologies. On the grounds that the wrapping cells and the central syncytium form a closely-knit anatomical unit, it is proposed that all of these elements together should be considered homologous to the epithelial intestine of the other turbellarian orders. A brief consideration of the phylogenetic implications of these results suggests that the construction of the central parenchyma in Convoluta sp. is a derived feature in relation to the other Turbellarian orders, and perhaps in relation to certain other Acoela.