A group of small sessile hemichordates that may be colonial (Rhabdopleura), pseudocolonial (Cephalodiscus), or solitary (Atubaria). Each individual, or zooid, lives inside a nonchitinous tube secreted by the protosome, except for Atubaria; an aggregation of zooids is called a coenecium and can vary in shape. The protosome, or oral shield, is disciform, closes the mouth ventrally, and secretes the tube. The protocoel has symmetrical pores at the base of the first pair of arms. The mesosome, or collar, has an anteroventral mouth and one to nine pairs of dorsally ciliated tentaculated arms, which are used to collect small organisms for food. The mesocoels extend into both arms and tentacles.