The oceanic or free-swimming forms of the Lucernaridæ resemble the Medusidæ by their bell-shaped umbrella, but differ from them by their internal structure, by the absence of a marginal veil, by the nature of their canal system and marginal bodies, and by their mode of development. The radiating canals, never less than eight in number, send off numerous branches, which form a very intricate network, and the vesicles and pigment-spots, here united into a single organ, termed the lithocyst, are each protected externally by a sort of hood, whence these jelly-fishes have been named "Stegonophthalmia," or "covered-eyed," by Forbes, to distinguish them from the naked-eyed "Gymnophthalmia," or Medusidæ. The Pelagidæ (Chrysaora), which form one of the divisions of this group, are simple, and have their margin surrounded with tentacles like the Medusidæ, while the Rhizostomidæ have no marginal tentacles, and consist of numerous polyps studding the trunks of a dependent tree. These animals have consequently no central mouth, but hundreds of little mouths all active for the welfare of the community. [Illustration: Oceanic forms of Lucernaridæ. _a._ _Rhizostoma pulmo._ _b._ _Chrysaora hysoscella._ _c._ Its lithocyst.--(All reduced.) ] The sessile Lucernaridæ differ from the other members of the order by the narrow disc or stalk which serves to fix their body when at rest. Their quadrangular mouth is in the centre of the umbrella expansion, and round the margin of the cup arise a number of short tentacles, disposed in eight or nine tufts in Lucernaria, and forming one continuous series in Carduella. [Illustration: Lucernalia auricula. (Natural size.)]