[Cnidaria] Marginal bulb developed on the umbrella margin without bearing tentacles. Some never develope tentacles, others can be the result of by tentacle reduction. It is necessary to distinguish between bulbs that are permanently without tentacles (permanent non-tentacular marginal bulbs, rudimentary marginal bulbs, as in Aequorea macrodactyla) and those bulbs on which a marginal tentacles will develop later on during medusan growth (developing tentacular marginal bulbs, as in Clytia and Malagazziidae).

Alternative form for non-tentacular marginal bulb : rudimentary bulb.