Fig. 2a–c.

Types of terminal cells in protonephridia. Terminal cells are broadly divided into several types. In most species, the filtration slits are an intercellular space between the terminal cells (a) or between the terminal cell and modulating tubular epithelial cell (b). In some animals, numerous rectangular filtration slits/fenestrations are opened at the tubular cytoplasmic wall of the terminal cell (c). T Terminal cell, M epithelial cell of modulating tubule. These schematic drawings are based on Kieneke et al. (2008)