Thecostraca is a class of typical crustaceans. There are 2077 species of Thecostraca, in 358 genera and 67 families. This class has been around since the guzhangian age. It includes groups like Barnacles.
Definition: number of currently described and taxonomically accepted marine species in this clade (Appeltans et al. 2012. The Magnitude of Global Marine Species Diversity. Current Biology 22, 2189–2202. http://dx.doi.org/10.1016/j.cub.2012.09.036)