The Stirodonta includes a number of extinct stem group lineages and has living taxa in three orders: Salenioida, Arbacioida
, and Phymosomatoida (Durham, 1966; Smith, 2005; Smith et al., 2006).
The five genera in which bindin was previously sequenced belong to two echinoid orders, the Echinoida and the Arbacioida
. These two orders contain only 10% of all extant echinoid species (Kier, 1977; Smith, 1984; Littlewood and Smith, 1995).