The Structure of the Epidermis in Didymorchis (Temnocephalida: Platyhelminthes)

Abstract

The epidermis of four species of Didymorchis ('Turbellaria', Temnocephalida) was studied using silver nitrate staining and electron microscopy. The epidermis is composed of 12-14 syncytia separated by lateral membranes. 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 modified 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 features of the epidermis are apomorphies of Didymorchis. Presence of borders between syncytia with the same structure cannot be explained on a functional 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. Diceratocephala) in which the separation of syncytia (with one exception) reflects functional differences between syncytia.