The millipede has two pairs of legs per segment (except for the first segment behind the head that has no complement at all, and the next one that has only one pair of legs). Each segment that has two legs is the result of two single segments combined together as one. Most millipedes have very long cylindrical bodies, though some diratakandorso-ventrally, while the thousandth leg of the pill is shorter and can roll into a ball, like a pillbug. Despite their name, the thousandth foot lacks 1,000 feet, though the rare species of Illacme plenipes has been up to 750. the species has between 36 and 400 feet. The class contains about 10,000 species in 13 orders and 115 families.