In many pennatulaceans it is a thickening in the upper part of the stalk. In hydroids it is a small swelling on the periphery of a hydromedusa. Growth of a new marginal tentacle is preceded by the appearance of a marginal bulb on which it then develops. In most species such bulbs do eventually bear tentacles, but in some species some bulbs permanently lack tentacles and these are called rudimentary bulbs. A basal bulb or tentacle bulb is one situated at the base of a tentacle arising from it.

Alternative forms for bulb : bulbs, marginal bulb.