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a class of deuterastomous animals of the phylum Hemichordata. Their development and the structure of their internal organs are similar to those of the Enteropneusta. Pterobranchs are small (up to 10 mm), stationary animals that develop through budding of colonies (except for the genus Atubaria). The body consists of a proboscis, a collar, and a trunk. The collar has paired, pinnate tentacles covered with cilia. The mouth opens on the ventral side of the body, between the proboscis and the collar. The intestine is looped; the anal opening is located at the posterior end of the body and is at the same level as the mouth. Pterobranchs include only a few species, which make up the orders Rhabdopleuroidea and Cephalodiscoidea. The former are found in the seas of northern Europe and East Asia, and the latter inhabit the waters of Antarctica.