The boring Bryozoa, here assigned to the order Ctenostomata Busk, 1852^, are an ethologically defined group of ecologically aberrant species which live totally "immersed" within solid calcareous substrata in the marine environment. Except for the operculum in the genus, Penetrantia Silen, 1946, only the lophophore ever extends above the surface of the inhabited object. (Of course, sexual reproduction involves the free-swimming larvae common to all bryozoans.) Probably because of their sheer abundance, the shells of gastropods and pelecypods are the substrata most commonly employed in post-Paleozoic seas, but other calcareous objects are also invaded — especially in the Paleozoic, when brachiopods and crinoids were the most readily available "hosts." (The term is placed in quotation marks because none of the bryozoans considered herein show any evidence of being parasitic; the boring habitus is solely a means of ensuring protection and anchorage while engaging in the heterotrophic, filter-feeding process characteristic of bryozoans.)