The microanatomy of a cubopolyp (polypoid stage of Cubomedusae) is described for the first time. The 0.5–1.0 mm long polyp ofTripedialia cystophora has an oral cone with special lip cells at the mouth. Next is a baggy calyx occasionally followed by a slender stalk. The basal region is surrounded by a thin periderm. A single row of tentacles is at the oral cone/calyx junction. The mesoglea is thin and non-cellular. The muscular system of the ectoderm is composed of smooth longitudinal epitheliomuscular cells in the oral cone, tentacle, stalk and calyx. The calyx ectoderm also sends longitudinal muscle fibers into the mesoglea. The mesogleal muscle fibers seem to contain paramyosin and perhaps are doubly innervated: one set of neurites for contraction and one for relaxation. A circular endodermal system of filaments, probably actin, is found in all regions. The tentacles have a solid core of a single row of endodermal cells capable of phagocytosis. The ectodermal tip is swollen with longitudinally aligned nematocysts. The distal part of the tentacle contains striated ectodermal myofibers. The nervous system is unique in having an endodermal/ectodermal nerve ring pair at the calyx/oral cone junction. Ganglion cells are not apparent. Presumed sense cells have complicated microvilli and no flagellar rootlet. A cell fitting the description of a neurosecretory neurone is especially prominent in the oral cone's endoderm. It has a major process reaching the coelenteron. Round macrogranular cells corresponding to the amoebocytes of the Scyphozoa and Anthozoa are found. There are no interstitial cells. The oral cone's flagellated endoderm is made up of mucous cells, cells with small dark granules, cells with large granules and rodlets in the cytoplasm, and a few absorptive cells. The calyx endoderm is very thick (120 μm) and is made up of flagellated absorptive, mucous and granular cells. Ingested food is transformed into basal droplets. 4 size and shape types of the microbasic eurytele category make up the cnidome. The largest nematocyst types are found at the tentacle's tip. Like a hydropolyp, the cubopolyp lacks gastral septa and is in other features radially symmetrical. Like a scyphopolyp, the cubopolyp has mesogleal muscles and no interstitial cells. Unique histological features are the nerve rings and tentacular striated muscles.