The members of this order are defined as being Lucertiarida in which the reproductive elements are developed in free zooids, produced by fission from attached Lucernaroids. The umbrella of the generative zooids is without marginal tentacles; and the polypites are "numerous, modified, forming with the genitalia a dendriform mass depending from the umbrella " (Greene).
The following is a brief summary of the life-history of a member of this extraordinary order (fig. 58), the illustration, however, representing the development of Aurelia, one of the Pelagidae, in which the phenomena are essentially the same. The embryo is a free - swimming, oblong, ciliated body, termed a "planula" (a), of a very minute size, and composed of an outer and inner layer, enclosing a central cavity. The planula soon becomes pear-shaped, and a depression is formed at its larger end. "Next, the narrower end attaches itself to some submarine body, whilst the depression at the opposite extremity, becoming deeper and deeper, at length communicates with the interior cavity. Thus a mouth is formed, around which may be seen four small protuberances, the rudiments of tentacula. In the interspaces of these four new tentacles arise; others in quick succession make their appearance, until a circlet of numerous filiform appendages, containing thread - cells, surrounds the distal margin of the 'Hydra - tuba' (b), as the young organism at this stage of its career has been termed by Sir J. G. Dalyell. The mouth, in the meantime, from being a mere quadrilateral orifice, grows and lengthens itself so as to constitute a true polypite, occupying the axis of the inverted umbrella or disc, which supports the marginal tentacles. The space between the walls of the polypite and umbrella is divided into longitudinal canals, whose relations to the rest of the organism, and, indeed, the whole structure of Hydra-tuba, closely resemble what may be seen in Lucernaria" (Greene, 'Manual of Coelenterata'). The Hydra-tuba thus constitutes the fixed "Lucernaroid," or the "trophosome" of one of the Rhizosto-midae. In height it is less than half an inch, but it possesses the power of forming, by gemmation, large colonies, which may remain in this condition for years, the organism itself being incapable of producing the essential elements of generation. Under certain circumstances, however, reproductive zooids are produced by the following singular process (fig. 58). The Hydra-tuba becomes elongated, and becomes marked by a series of grooves or circular indentations, extending transversely across the body, from a little below the tentacles to a little above the fixed extremity. At this stage the organism was described as new by Sars, under the name "Scyphistoma." The annula-tions or constrictions go on deepening and become lobed at their margin, till the Scyphistoma assumes the aspect of a pile of saucers, arranged one upon another with their concave surfaces upwards. This stage was described by Sars under the name of "Strobila" (c). The tentacular fringe which originally surrounded the margin of the Hydra-tuba now disappears, and a new circlet is developed below the annulations, at a point a little above the fixed extremity of the Strobila (c). "The disc-like segments above the tentacles gradually fall off, and, swimming freely by the contractions of the lobed margin which each presents, they have been described by Eschscholtz as true Medusidae under the name of Ephyrae (d)." Each Ephyra, however, soon shows its true nature by becoming developed into a free-swimming reproductive body, usually of large size, with umbrella, hooded lithocysts, and tentacles, constituting, in fact, a Steganophthalmate Medusa. The reproductive zooid now swims freely by the contraction of its umbrella, and it eats voraciously and increases largely in size. The essential elements of generation are then developed in special cavities in the umbrella, and the fertilised ova, when liberated, appear as free-swimming, ciliated "planulae," which fix themselves, become Hydra-tubae, and commence again the cycle of phenomena which we have above described.
Fig. 58. - Development of Aurelia, one of the Lucernarida. a Ciliated free-swimming embryo, or "planula;" b Hydra-tuba; c Hydra-tuba in which fission has considerably advanced, and the "Strobila" stage has been reached ; d Hydra-tuba in which the fission has proceeded still further, and a large number of the segments have been already detached to lead an independent existence.
As regards the size of these reproductive zooids as compared with the organism by which they are given off, it may be mentioned that the umbrella of Cyanea arctica has been found in one specimen to be seven feet in diameter, with tentacles more than fifty feet in length, the fixed Lucernaroid from which it was produced not being more than half an inch in height.
As regards the special structure of these gigantic reproductive bodies, considerable differences obtain between the Rhizo-stomidae and that section of the Pelagidae in which this method of reproduction is employed. In the Pelagidae, namely, the generative zooids possess a general, though chiefly mimetic, resemblance both to the genuine Medusidae and to the free-swimming medusiform gonophores of so many of the Hydrozoa, and they have the following structure. Each (fig. 59) consists of a bell shaped, gelatinous disc, the "umbrella," from the roof of which is suspended a large poly-pite, the lips of which are extended into lobed processes often of considerable length, "the folds of which serve as temporary receptacles for the ova in the earlier stages of their development." The poly-pite - manubrium or proboscis - is hollowed into a digestive sac, which communicates with a cavity in the roof of the umbrella, from which arises a series of radiating canals, the so-called " chyl-aqueous canals." These canals, which are never less than eight in number, branch freely and anastomose as they pass towards the periphery of the umbrella, while the entire series is connected by a circular marginal canal. This, in turn, sends tubular processes into the marginal tentacles, which are often of great length. Besides the tentacles, the margin 0f the umbrella is furnished with a series of peculiar bodies, termed "lithocysts," each of which is protected by a sort of process or hood derived from the ectoderm, and consists essentially of a combined "vesicle" and "pigment-spot," such as have been described as occurring separately in the Medu-sidae. These marginal bodies likewise communicate with the chylaqueous canals. The reproductive elements "are lodged in saccular processes of the lower portion of the central cavity, immediately above the bases of the radiating canals, and being usually of some bright colour, form a conspicuous cross shining through the thickness of the disc " (Greene).