THE STRUCTURE OF THE EPIDERMIS IN DIDYMORCHIS (TEMNOCEPHALIDA, PLATYHELMINTHES)

The epidermis of four species of Didymorchis ('Turbellaria', Temnoceph alida) was studied using silver nitrate staining and electron microsco py. The epidermis is composed of 12-14 syncytia separated by lateral m embranes. The organisation of the epidermal mosaic is almost identical for all species studied. Neighbouring syncytia (or groups of syncytia ) differ in the presence (or absence) and density of locomotory cilia, in the structure of the cytoplasm, and in the abundance and nature of the gland ducts that penetrate them. The dorsal syncytia differ from the ventral ones in the form of the electron-dense inclusions in the m odified mitochondria present throughout the epidermis. Multisyncytial organisation of the epidermis supports the placement of Didymorchis in the Temnocephalida rather than in the Dalyellioida. Three other featu res of the epidermis are apomorphies of Didymorchis. Presence of borde rs between syncytia with the same structure cannot be explained on a f unctional basis. We suggest that it represents a relatively primitive stage in the evolution of the multisyncytial epidermis. In this regard Didymorchis is different from more specialised temnocephalids (e.g. D iceratocephala) in which the separation of syncytia (with one exceptio n) reflects functional differences between syncytia.