Philocelis cellata has a strengthened bursa mouthpiece which is arranged in front of the male copulatory organ. The main components of the bursa mouthpiece are numerous ring-shaped bursa mouthpiece cells whose central parts contain strengthened elements forming a tube around the sperm duct. Each of the peripheral areas of the bursa mouthpiece cells is separated by similarly ring-shaped gap cells. The end of the bursa mouthpiece towards the bursa is formed by a so-called

sorting apparatus

which consists of different cells; opposite the bursa the sperm duct ends in a globe-shaped sperm vestibule. The bursa mouthpiece is differentiated successively, beginning at the distal part at the bursa and proceeding proximally.
Keywords Plathelminthes - Acoela - ultrastructure - bursa mouthpiece