Morphology and systematics of some interior-walled cheilostome Bryozoa P. L. Cook and P. J. Chimonides Department of Zoology, British Museum (Natural History), Cromwell Road, London SW7 5BD Contents Synopsis 53 Introduction 53 Material and methods 54 Measurements and abbreviations used 57 Key to species described 58 Systematic section 58 Family Euthyrisellidae Bassler 58 Family Didymosellidae Brown 68 Discussion 81 Summaries in French and German 83 Acknowledgements 85 References 85 Synopsis The colony structure and zoodial morphology found in two cryptocystidean cheilostome bryozoan Families, the Euthyrisellidae and the Didymosellidae are described. All calcified walls are interior, and colony-wide (extrazooidal) basal coeloms are present in nearly all species. In the Family Euthyrisellidae, interzooidal visceral-to-hypostegal communication complements or even replaces the more usual, intrazooidal kind, which is that found in the Family Didymosellidae. In the Euthyrisellidae an extension of the hypostegal coelom on the basal side of the calcified frontal shield is present in most species. One new species, Tropidozoum burrowsi, is introduced. The Family Didymosellidae has a fossil record extending from the Middle Eocene to the Recent, and a wide Tertiary-to-Recent distri-budon. One new Recent species, Didymosella pluma, is introduced, and Tubiporella boninensis Borg is redefined. Tubiporella levinseni Borg is redescribed and assigned to the umbonuloid ascophoran genus Reptadeonella. Introduction The calcified and uncalcified body walls defining the member zooids of bryozoan colonies may conveniently be regarded as belonging to two kinds. The first is the exterior, limiting or boundary wall, which is primarily cuticular and which, if calcified, almost invariably has an outermost layer of cuticle throughout ontogeny. This kind of wall is actually or potentially capable of dividing the internal coelomic tissues of zooids from the environment. Zooids delimited by this kind of wall develop as uncalcified buds by expansion and extension of coelomic tissue through septulae in the calcified wall of an existing zooid. The boundary wall of cuticle and epidermis expands intussusceptively as growth proceeds. The earliest known, Late Jurassic cheilostomes had zooidal walls almost entirely of this kind (see Boardman & Cheetham, 1973), and large numbers of species now living have the same, or similar zooidal structure and budding pattern. Many lateral vertical walls of contiguous Bull. Br. Mus. nat. Hist. (Zool.) 41 (2) 53-89. Issued 24 September 1 98 1 53
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