Fig. 4.

Electron micrographs of structures with phylogenetic significance. a Statocyst of a hatchling of Isodiametra pulchra with two parietal cells (p) and a lithocyte (l). b Sperm of Convoluta niphoni (Convolutidae) with axial microtubules (white arrow) and axonemes without central microtubules (white arrowheads). c Extrusion apparatus of Convolutriloba hastifera consisting of a sagittocyst (black arrowhead) and a wrapping muscle mantle. Abbreviations: m muscle mantle; n nucleus of muscle mantle; rb refractive body; p parietal cell. Scale bars: a 4 μm; b 0.5 μm; c 2 μm