TEM micrographs of S. yangi sperm tail region. A Longitudinal section of sperm tail, showing axoneme (ax) and mitochondrial derivative (md) B Cross-section through the tail region, showing axoneme (ax) and mitochondrial derivatives (md) with crystalline region (cry) C Magnified cross-section of axoneme (ax), showing axoneme with a normal 9 + 9 + 2 arrangement of microtubules, i.e., 9 accessory microtubules (am), 9 double microtubules (dm) and 2 central microtubules D Cross-section of the terminal end of the sperm tail, showing paired mitochondrial derivatives (md) and axoneme (ax) with 9 accessory microtubules and 9 double microtubules left E Cross-section of the terminal end of the sperm tail, showing parts of microtubules of axoneme left. Scale bars: 500 nm (A), 200 nm (B), 100 nm (C–E).