[Hydromedusae] Club-like organs for orientation and equilibration, growing out of the umbrella margin in the fashion of a tentacle; each is formed by an endodermal axis originating from the circular canal and covered by the umbrellar ectoderm. In their distal portion there are one or more large endoderm cells (lithocytes), each containing a solid concretion (statolith); these structures are termed free ecto-endodermal statocysts (e.g., Narcomedusae, some Limnomedusae and Trachymedusae; Statocyst types, Pegantha triloba 2), but in some species the sensory clubs are entirely enveloped by an ectodermal vesicle and are then called closed ecto-endodermal statocysts (e.g., some Limnomedusae and Trachymedusae) (Bell margin 4, Side view P. rubiginosa, Statocyst types).