Microanatomy of the bell rim ofAurelia aurita(Cnidaria: Scyphozoa)

Abstract

The bell rim of Aurelia aurita is described in detail. This medusa has usually eight marginal sensory complexes and many tentacle chambers, the latter made up of a roof, sides (lappets), and a floor. The sole contents of a tentacle chamber are a few bladelike tentacle bases. The distal tentacle is narrow and moniliform and grooved on the adoral side with a folded sheet of muscle fibers on the deep side. Between the underside of the tentacle chamber's floor and a more adoral circumferential flange (pseudovelarium) lies a ciliated food groove lined with a thick amuscular epithelium containing ordinary surface cells, mucous cells, and presumed digestive cells. The subumbrella is bilayered, with an outer epithelial layer that sends thin cytoplasmic sheets to the mesoglea; deeper is a layer of myocytes each with a striated circular myofibril. Towards the periphery, and before the pseudovelarium, is a rim of smooth radial myocytes overlaid with an epithelial layer. The adoral side of the pseudovelarium has smooth radial epitheliomuscular cells. Where the pseudovelarium attaches to the subumbrella, neuromuscular cords traverse the mesoglea to join the adoral side of the tentacles. Along the food groove and midway between rhopalia is a pocket in the pseudovelarium, the food pouch, which collects plankton. The pararhopalial region of the rim is different in that the floor of the tentacle chamber is short or absent; a modified intertentacular partition can sometimes form a protuberance on the aboral side of the pseudovelarium; and the pseudovelarium takes a right-angled turn outwards near the rhopalium to form the thin part of the rhopalial hood. This type of rim is specialized for the concentration and, perhaps, the early digestion of plankton.