The ultrastructure of the protonephridial flame cell of the freshwater planarian Bdellocephala brunnea.

The protonephridial flame cell of the freshwater planarian Bdellocephala brunnea was studied by electron microscopy. This basketlike cell with luminal cilia and fenestrae is known as a cyrtocyte. Careful examination revealed the actual continuity of the fenestrae over the surface of the cell and the microvillous nature of thin cytoplasmic rods composing the fenestrae. On the basis of these findings, a new interpretation was given for the threedimensional configuration of a single cell: this cell is essentially stellate in shape, with several irregular cytoplasmic processes adjoining one another, thus bearing a basic resemblance to the podocyte of a mammalian renal corpuscle. The cytoplasmic features strongly suggest its great capability of reabsorbing substances from the filtrate and of carrying out intracellular digestion. Some additional structural specialization of the fenestra, serving for enlargement of the filtration surface, was observed.