[Hydromedusae] In most medusae, a dilated portion of the proximal part of a marginal tentacle, next to the umbrellar margin (Medusa 2, Bell margin, Bell margin 2, Bell margin 3). The marginal tentacle bulbs contain a cavity in communication with the circular canal and with the tentacular cavity of the tentacles when they are hollow. They are variable in shape, most bulbs are simple but in some medusae they are compound, and several tentacles may arise from a single tentacular bulb (e.g., Bougainvillia). During the growth of a medusa a new marginal tentacle is normally preceded by the formation of a bulb on which it develops (see marginal bulb). In some medusae, however, there are no tentacular bulbs (e.g., Calycopsidae, Gossea, Proboscidactyla).