Class of minute, fixed, colonial, deep-sea organisms which secrete an external cuticular skeleton in which they are housed (e.g. Rhabdopleura). Pterobranchs may be the nearest living relatives of the graptolites (Graptolithina). The mesosoma is small but carries one or more pairs of arms bearing tentacles (the lophophores). The metasoma consists of a long stalk (peduncle) by which the individual is attached. The branchial apparatus is rudimentary.