What is "sessilia"

Sessilia is an order of barnacles, comprising the barnacles without stalks, or acorn barnacles. They form a monophyletic group and are probably derived from stalked barnacles. The order is divided into three suborders. Brachylepadomorpha contains a single family, Neobrachylepadidae, while Verrucomorpha contains two families, Verrucidae and Neoverrucidae. The remaining twelve families are in the suborder Balanomorpha.