Sessilia is an order of barnacle

Barnacle

A barnacle is a type of arthropod belonging to infraclass Cirripedia in the subphylum Crustacea, and is hence related to crabs and lobsters. Barnacles are exclusively marine, and tend to live in shallow and tidal waters, typically in erosive settings. They are sessile suspension feeders, and have...

s, comprising the barnacles without stalks, or acorn barnacles. They form a monophyletic group

Monophyly

In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...

 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

Balanomorpha

Balanomorpha is a suborder of barnacles, containing familiar acorn barnacles of the seashore. It contains the following families:*Archaeobalanidae Newman & Ross, 1976*Balanidae Leach, 1817*Bathylasmatidae Newman & Ross, 1971*Catophragmidae Utinomi, 1968...

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