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1. (class Turbellaria, subclass Archoophora) An order of platyhelminth worms (Platyhelminthes) that have no gut cavity or pharynx.

2. (class Gastropoda, subclass Opisthobranchia) An order of molluscs that possess no shell, mantle cavity, or gills. Respiration is carried out by branchiae. Dorsal outgrowths are quite common, and all have undergone complete detorsion. Benthic and planktonic forms occur. There is only one fossil family recorded, which appeared in the Eocene. According to some authorities, the taxonomic use of ‘Acoela’ as a molluscan order should be discouraged. It represents an attempt to unite the Notaspidea and Nudibranchia.

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