This order is defined as including Lucernarida which possess a single polypite only, and an umbrella with marginal tentacles. The reproductive elements "are developed in a free umbrella, which either con-stituies the primitive hydrosoma, or is produced by fission from an attached Lucernaroid" (Greene).
Two types, therefore, exist in the Pelagidae. The one type is represented by a fixed "trophosome," resembling Lucen naria, but distinguished from it by the fact that the generative elements are not developed in the primitive hydrosoma, but in a free "gonosome," which is produced for the purpose. The second type, represented by Pelagia itself, is permanently free, thereby differing from Lucernaria, which it approaches, on the other hand, in the fact that its generative elements are produced in its own umbrella without the intervention of free generative zooids. Pelagia, however, differs considerably in structure from Lucernaria, and in all essential characters is not anatomically separable from a Steganophthalmate Medusid. The process of reproduction as displayed in the first section of the Pelagidae will be considered when treating of that of the Rhizostomidae, there being no important difference between the two, except as concerns the structure of the generative zooids.
Fig. 57. - Lucernariadae. Lucernaria auricula attached to a piece of sea-weed (after Johnston).