This order includes those Lucernarida which have only a single polypite, are fixed by a proximal hydrorhiza, and possess short tentacles on the margin of the umbrella. The reproductive elements " are developed in the primitive hydrosoma without the intervention of free zooids" (Greene).
In Lucernaria (fig. 57), which may be taken as the type of the order, the body is campanulate or cup-shaped, and is attached proximally at its smaller extremity by a hydrorhiza, which, however, like that of the Hydra, is not permanently fixed. When detached, the animal is able to swim with tolerable rapidity by means of the alternate Contraction and expansion of the umbrella. Around the margin of the umbrella are tufts of short tentacular processes, and in its centre is a polypite with a quadrangular four-lobed mouth. "In transverse section the polypite may be described as somewhat quadrilateral, with a sinuous outline which expands at its four angles to form as many deep longitudinal folds, within which the simple genera-tive bands are lodged" (Greene). Wide longitudinal canals are formed by septa passing from the walls of the polypite to the inner surface of the cup, and a circular canal runs immediately beneath the insertion of the tentacles. The reproductive elements are produced within the body of Lucernaria itself, without the intervention of any generative zooid.