Introduction

Palombiella stephensoni (Palombi, 1938) is a marine triclad (Phylum Platyhelminthes, Class Turbellaria, Order Tricladida) of the family Bdellouridae. The morphology of the body wall and musculature of this animal as seen by light microscopy of histological preparations has been described (Wineera, 1969). In the present study the methods of investigation employed allowed a much better characterisation of certain of the body wall constituents. The tissues of flatworms are notoriously difficult subjects for study, and the methods of "conventional" histology, such as the examination of stained paraffin sections frequently give poor results. This is particularly true for the parenchyma. In this tissue nuclei can be seen, but cell boundaries are indistinct or invisible. Extraction of fats during tissue processing, and distortion of tissues causes the appearance of irregular vacuoles which may be mistaken for intercellular spaces (Pedersen, 1961a). The walls of such vacuoles may appear as connective tissue fibres. Moreover, as Pedersen further states, none of the classical page 2 staining methods gives meaningful histological pictures, and the need for thin sections precludes the use of celloidin or frozen sections. These factors contributed to the parenchyma being called syncytial.

The methods of tissue processing employed for electron microscopy have recently (Pedersen, 1961a; Richardson, Jarett and Finke, 1960) been found to be excellent for the study of thin (0.25 to 3μ.) sections by light microscopy. One of these methods is employed in this study, and promises to be an important tool in future cytological and histological work.