This extinct group of cone-shaped organisms has defied easy classification. The calcareous skeletons of these organisms are commonly found forming simple reef structures in Cambrian rocks, and are common in rocks of that age. The basic morphology consists of two nested, perforate cones connected by a series of septa. Recent work on the biomechanics of water flow through this structure has revealed that archaeocyatids operated much like sponges, and on that basis some workers have classified them with sponges. This class of cnidarians has true alternation of generation in its reproductive cycle.
Many groups secrete a chitinous skeleton in the polypoid stage.
In some groups the polypoid stage is colonial and secretes a calcareous skeleton. Scyphozoans emphasize the medusoid form, rarely if ever producing polyps. Most of these free swimming organisms have no hard skeleton, but the group still possesses a fossil record going back to the Precambrian. Note the four-fold symmetry in the medusa. Like the archaeocyatids and the stromatoporoids, this extinct group has defied easy classification.