The organisms included in this order have often been separated as a distinct sub-class of the Hydrozoa; but they are, perhaps, best regarded as a mere order of the Hydroid Zoophytes, characterised by the fact that the hydrosoma is free-swimming and oceanic, consisting of a single swimming - bell ("nectocalyx"), from the roof of which is suspended a single poly-pite. A system of canals is developed in the walls of the swimming-bell, and the reproductive organs are processes of the sides of these canals or of the walls of the polypite.

The Medusidae comprise most of the smaller organisms commonly known as Jelly-fishes or Sea-nettles, the last name being derived from the property which some of them possess of severely stinging the hand, this power being due to the presence of numerous thread-cells. As employed by modern naturalists, the order is very much restricted, many of its members having been shown to be really the free generative buds of other Hydrozoa. As used here, it corresponds to part of the Gym-nophthalmate Medusae of Professor E. Forbes, the Steganoph-thalmate Medusae of the same author being now placed in the sub-class Lucernarida.

* The old sub-class of the Acalephae contained the Gymnophthalmate Medusae (= the Discophora), and the Steganophthalmate Medusa (= the Lucernarida in part), the two being placed in a single order under the name of Pulmograda. The Acalephae also contained the Ctenophora and the Calycophoridae and Physophoridae, of which the former constituted the order Ciliograda, whilst the two latter made up the order Physograda. The Ctenophora, however, are now generally placed amongst the Actinozoa, whilst the Calycophoridae and Physophoridae constitute the Hydrozoal subclass Siphonophora.

The hydrosoma of one of the Medusidae (= a Gymnophthal-mate Medusa) is composed of a single, gelatinous, bell-shaped swimming organ, the "nectocalyx" or "disc," from the roof of which a single polypite is suspended (figs. 52, 53). The interior of the nectocalyx is often called the "nectosac," and the term " codonostoma " has been proposed to designate the open mouth of the bell. The margin of the nectocalyx is produced inwards to form a species of shelf, running round the margin of the mouth of the bell, and termed the "veil" or "velum," by the presence of which the nectocalyx is distinguished from the somewhat similar "umbrella" of the Lucernarida. The endodermal lining of the central polypite or "manubrium" (sometimes called the "proboscis") is prolonged into four, sometimes more, radiating canals, which run to the periphery of the nectocalyx, where they are connected by a circular canal which runs round its circumference, the whole constituting the system of the "nectocalycine canals " (often called the "chylaqueous" or "gastro-vascular canals"). From the circumference of the nectocalyx depend marginal tentacles, which are usually hollow processes, composed of both ectoderm and endoderm, and in immediate connection with the canal system. Also round the circumference of the nectocalyx are disposed certain "marginal bodies," of which two kinds may be distinguished. Of these the first are termed "vesicles," and consist of rounded sacs lined by epithelium, and containing one or more solid, motionless concretions - apparently of carbonate of lime - immersed in a transparent fluid. The second class of marginal bodies, variously termed "pigment-spots," "eye-specks," or "ocelli," consists of little aggregations of pigment enclosed in distinct cavities. The "vesicles" are probably rudimentary organs of hearing, and possibly the eye-specks are a rudimentary form of visual apparatus. The oral margin of the polypite may be simple, or it may be produced into lobes, which are most frequently four in number. The essential elements of generation are produced in simple expansions either of the wall of the manubrium or of the radiating nectocalycine canals.

Order VI MEDUSIDAE or Hydromedusidae Acalephae in  63Fig. 52.*   Morphology of Medusidae. a A Medusoid (Thaumantias) seen in profile, showing the central polypite, the radiating and circular gonocalycine canals, the marginal vesicles and tentacles, and the reproductive organs; b The same viewed from below. The dotted line indicates the margin of the velum.

Fig. 52.* - Morphology of Medusidae. a A Medusoid (Thaumantias) seen in profile, showing the central polypite, the radiating and circular gonocalycine canals, the marginal vesicles and tentacles, and the reproductive organs; b The same viewed from below. The dotted line indicates the margin of the velum.

* The form here figured (fig. 52), though in all respects anatomically identical with the true Medusa, and originally described as such, is now known to be in reality the medusoid bud of a fixed Hydroid. It illustrates the structure and form of the Medusae, however, just as well as though it were completely independent in its development.

Fig. 53.   Trachynema digitale, a naked eyed Medusa, female, enlarged. (After A. Agassiz.) p Manubrium or central polypite; t One of the tentacles ; c One of the gastro vascular canals; o One of the ovaries.

Fig. 53. - Trachynema digitale, a naked-eyed Medusa, female, enlarged. (After A. Agassiz.) p Manubrium or central polypite; t One of the tentacles ; c One of the gastro-vascular canals; o One of the ovaries.

From the above description it will be evident that the Medusa is in all essential respects identical in structure with the free-swimming generative bud or gonophore of many of the fixed and oceanic Hydrozoa. Indeed, a great many forms which were previously included in the Medusidae. have now been proved to be really of this nature; and it is only in a comparatively small number of Medusa that the direct mode of development which alone would entitle them to be ranked as independent organisms has been observed. The Trachytiemidae, AEginidae, and Geryonidae appear, however, to be directly developed from the ovum, and are therefore properly placed in the present order; while we may temporarily include here a number of Medusoid forms, the development of which is at present unknown.

As to the development of these true Medusidae, little is known for certain. It appears, however, that in Trachynema, AEginopsis, and other genera, the embryo is directly developed into a form resembling its parent, without passing through any intermediate changes of form. It is hardly necessary to remark that this is not the case with the embryos of a medusiform gonophore, these being developed into the sexless Hydrozoon by which the medusoid was produced. It has also been shown that various of the true Medusae (Cunina, AEgineta, &c), have the power of producing new forms like themselves by a process of budding, the phenomena attendant upon this being sometimes of great interest.

In this connection, allusion may be made to the long-known fact that certain "medusiform gonophores" are likewise capable of producing independent forms directly resembling themselves by a process of gemmation, and not by one of true reproduction. Technically these are called "tritozooids," as being derived from organisms which are themselves but the generative zooids of another being. This singular phenomenon has been observed in various medusiform gonophores (e.g., Sarsia gemmifera), the buds springing in different species from the gonocalycine canals, from the tentacles, or from the sides of the polypite or manubrium.

The "naked-eyed" Medusae and their allies the "medusiform gonophores," though mostly very diminutive in point of size, are exceedingly elegant and attractive when examined in a living condition, resembling little bells of transparent glass adorned here and there with the most brilliant colours. They occur in their proper localities and at proper seasons in the most enormous numbers. They are mostly phosphorescent, or capable of giving out light at night, and they appear to be one of the principal sources of the luminosity of the sea. It does not seem, however, that they phosphoresce, unless irritated or excited in some manner.