Both genera are characterized by their habit of secreting a tubular gelatinoid investment, the "coenoecium," composed of a number of superposed lamellae, doubtless the result of its intermittent secretion, mainly though perhaps not exclusively, by the proboscides of the zooids.
In Cephalodiscus the coenoecium is more massive, and may contain a continuous irregular cavity in which the zooids live (Demiothecia), or may be secreted in such a way that each zooid has its own independent tube (Idiothecia, Orthoecus).
While in Rhabdopleura the buds remain in organic continuity with the parent, in Cephalodiscus they become free at an early stage, and the coenoecium accordingly contains a number of separate individuals.
In the living Cephalodiscus a zooid can crawl by means of its proboscis over the gelatinous processes of the outer side of the coenoecium, a position which it can assume owing to the very great extensibility of the stalk, the proximal suctorial end of which remains attached to the inner surface of some part of the coenoecium (Andersson, 1907).