Cyclostomatida, or cyclostomata (also known as cyclostomes), are an ancient order of stenolaemate bryozoans which first appeared in the Lower Ordovician. It consists of 7+ suborders, 59+ families, 373+ genera, and 666+ species. The cyclostome bryozoans were dominant in the Mesozoic; since that era, they have decreased. Currently, cyclostomes seldom constitute more than 20% of the species recorded in regional bryozoan faunas. Taxonomy Traditionally, cyclostomes have been divided into two groups according to the skeletal organization. In free-walled (or double-walled) cyclostomes, the exterior frontal walls of the zooids are uncalcified; autozooids have either a polygonal aperture bounded by vertical interior walls, or a subcircular aperture in species with kenozooids filling the spaces between the autozooids. By contrast, fixed-walled (or single-walled) cyclostomes have much of the exterior frontal wall calcified; autozooids normally have a subcircular aperture located at or close to the frontal wall. Anatomy Among cyclostome bryozoans, the calcitic skeleton is usually lamellar, consisting of tabular or lath-like crystallites stacked like tiles at a low angle to the wall surface. Cheilostome bryozoans may exhibit a similar ultrastructure but more commonly have fibrous skeletons consisting of needle-like or bladed crystallites oriented almost perpendicular to wall surfaces. The skeletal parts of individual feeding zooids - autozooecia - are typically long, curved tubes with terminal apertures which are either circular or polygonal in shape. Colonies vary greatly in form according to species. Many cyclostomes have encrusting colonies, firmly cemented to hard substrates such as rocks and shells. These usually grow as subcircular sheets, spots or pimples, or systems of ramifying branches. Most erect cyclostomes develop bushy colonies with narrow, bifurcating branches. Feeding zooids are borne either evenly around the branch circumference or are absent from one face. New zooids are generally formed in distinct budding zones, for example, around the outer circumference of subcircular encrusting colonies, or at the tips of the branches in ramifying encrusting and bushy erect colonies. All post-Palaeozoic cyclostomes have interzooidal connections via pores in the vertical walls separating adjacent zooids. In the fixed-walled forms, these narrow pores constitute the only coelomic connection, because the vertical walls contact and fuse with the calcified, exterior frontal walls. However, in free-walled forms, the frontal walls are uncalcified, and no contact is made with the vertical walls, creating a wider coelomic connection around the distal ends of the vertical walls. Polymorphism of zooids is less conspicuous than in cheilostomes. However, almost all