Sub-class IV. Lucernarida (Acalephae, in part)

The members of this sub-class may be defined as Hydrozoa "whose hydrosoma has its base developed into an ' umbrella,' in the walls of which the reproductive organs are produced" (Greene).

A large number of forms included in the Lucernarida were described by Edward Forbes under the name of Steganophthal-mate Medusa, being in many external characters closely similar to the Medusidae. These "hidden-eyed" Medusae are familiar to every one as "sea-blubbers" or "sea-jellies," and they occur in great numbers round our coasts during the summer months. The resemblance to the little jelly-fishes is especially strong between the disc or "nectocalyx" of the true Medusidae and the "umbrella" of the Lucernarida, the latter being often a bell - shaped swimming organ, with marginal tentacles, and containing one or more polypites. These analogous structures (figs. 53 and 59) are, however, distinguished as follows : 1. The "umbrella" of the Lucernarida is never furnished with a "velum," as is the nectocalyx of the Medusidae. 2. The radiating canals in the former are never less than eight in number, and they send off numerous anastomosing branches, which join to form an intricate network; whereas in the latter they are rarely more than four in number, and though they may subdivide, they do not anastomose. 3. In the place of the separate and unprotected "vesicles" and " ocelli" of the Medusidae, the marginal bodies of the Lucernarida consist of these bodies combined together into single organs, which are termed "lithocysts," and which are protected externally by a sort of hood.

The Lucernarida admit of being divided into three orders - viz., the Lucernariadae, the Pelagidae, and the Rhizostomidae.