Clue to the Origin of Anucleate Flame Bulbs in Some Flatworms

Abstract

Two of three investigated species of kalyptorhynch turbellarians showed evidence of degenerating nuclei in voluminous terminal flame bulbs (protonephridia). No cell boundaries separated the flame bulbs from the proximal canals. This situation may represent the developmental and evolutionary pathway from the plesiomorphic nucleated flame bulbs of many flatworm taxa to the apomorphic, anucleate flame bulbs found in the majority of rhabdocoel taxa that have been studied. Additional key words: Platyhelminthes, Rhabdocoela, protonephridia, autolysis The structure of the protonephridial system has been studied in a number of platyhelminth taxa, and differences have been used in phylogenetic analyses (see reviews by Rohde 1990, 1991). In most groups protonephridia comprise many terminal flame bulbs connected to a duct or canal system that empties through one or more excretory pores. The flame consists of a number of cilia that beat together, giving the appearance (in the light microscope) of a flickering flame. Cilia of the flame are located inside a cup-shaped structure with filtration slits through its wall. The flame bulbs found in most groups investigated so far are discrete cells with a nucleus near the flame and with cell membranes separating each terminal cell from the adjacent canal region. Exceptions to this construction are found in Lecithoepitheliata and many members of