The midpiece of spermatzoa in nearly all vertebrates is enclosed in a sheath of mitochondria, which provide a ready source of ATP necessary for sperm motility. Here, a longitudinal section through spermatozoa of the dormouse, Glis glis, shows elongate mitochondria arranged end to end in a helix, an organization common in mammals. This image by David Phillips is Figure 261 from Chapter 7 (Mitochondria) of 'The Cell, 2nd Ed.' by Don W. Fawcett M.D. A PDF copy of the accompanying chapter is available on the ASCB’s BioEDUCATE website.