Cnidaria of exclusively polypoid form, no medusoid stage ever occurring. The body of an individual polyp consists of a hollow column whose cavity or coelenteron is divided into chambers by radially arranged mesenteries. A transverse oral disc terminates the distal end of the column and from the mouth at its centre a tubular, throat-like actinopharynx leads into the coelenteron. A number of hollow tentacles arise from the oral disc, their cavities being continuous with the coelenteron. The basal part of the polyp varies in structure according to its function. Colonial and solitary forms occur, with or without various hard skeletal elements. The cnidom includes spirocysts, atrichs, holotrichs, b- and p-mastigophores, in various combinations in different taxa.