The Cyclostomata are numerous in Palaeozoic rocks, but attained a specially predominant position in the Cretaceous strata, where they are represented by a prpfusion of genera and species; while they still survive in considerable numbers at the present day.
The body-wall is extensively calcified in the Cyclostomata and in most Cheilostomata, which may form elegant network-like colonies, as in the unilaminar genus Retepora, or may consist of wavy anastomosing plates, as in the bilaminar Lepralia foliacea of the British coasts, specimens of which may have a diameter of many inches.
The mechanism of protrusion in the Cyclostomata is a subject which requires further examination.
In the Cyclostomata, further distinguished by the cylindrical or prismatic form of their highly calcified zooecia, the orifice is typically circular, without any definite closing organ.