the Physophorida obey the same general law as the Calycophorida.

In Physophora the hydrosoma consists of a filiform cœnosarc, which bears the polypites and their appendages, and dilates proximally into a pneumatophore. Below this point the coenosarc bears a double row of nectocalyces, which are channelled on their inner faces to allow of their attachment to the coenosarc. There are no hydrophyllia, but there is a series of "hydrocysts" on the proximal side of the polypites.

Physalia, or the Portuguese man-of-war (fig. 47, a) is composed of a large, bladder-like, fusiform float" or pneumatophore-sometimes from eight to nine inches in length-upon the under surface of which are arranged a number of polypites, together with highly contractile tentacles of great length, "hydrocysts," and reproductive organs. Physalia is of common occurrence, floating at the surface of tropical seas; and fleets of it are not uncommonly driven upon our own shores.

In Velella (fig. 47, b) the hydrosoma consists of a widely-expanded pneumatophore of a rhomboidal shape, carrying upon its upper surface à diagonal vertical crest. Both the horizontal disc and the vertical crest are composed of a soft marginal "limb," and a central more consistent "firm part." "To the distal surface of the firm part of the disc are attached the several appendages, including—1, a single large polypite, nearly central in position; 2, numerous small gonoblastidia, which resemble polypites, and are termed 'phyogemmaria; and, 3, the reproductive bodies to which these last give rise. The tentacles are attached, quite independently of the polypites, in a single series along the line where the firm part and limb of the disc unite. There are no hydrocysts, nectocalyces, or hydrophyllia. On all sides the limb is traversed by an anastomosing system of canals, which are ciliated, and communicate with the cavities of the phyogemmaria and large central polypite" (Greene). Velella is about two inches in length by one and a half in height. It is of a beautiful blue colour and semi-transparent, and it floats at the surface of the sea, with its vertical crest exposed to the wind as a sail.

CHAPTER X.

DISCOPHORA.

SUB-CLASS III. DISCOPHORA (Acalepha* in part).—Since the sub-class contains only a single order, that of the Medusida, a

The old sub-class of the Acalephe contained the Gymnophthalmate Medusa (the Discophora), and the Steganophthalmate Medusa (= the Lucernarida in part), the two being placed in a single order under the name of Pulmograda. The Acalephe also contained the Ctenophora and the Calycophorida and Physophoride, 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 Calycophorida and Physophorida constitute the Hydrozoal sub. class Siphonophora.

single definition necessarily suffices for both. The Medusida are defined as "Hydrozoa whose hydrosoma is free and oceanic, consisting of a single nectocalyx, from the roof of which a single polypite is suspended. The nectocalyx is furnished with a system of canals. The reproductive organs are as processes either of the sides of the polypite or of the nectocalycine canals" (Greene).

The Medusida comprise most of the 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, and it is by no means improbable that it will ultimately be entirely done away with, very 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 Gymnophthalmate Medusa of Professor E. Forbes, the Steganophthalmate Medusa of the same author being now placed in the sub-class Lucernarida.

The hydrosoma of one of the Discophora ( a Gymnophthalmate 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 (fig. 48). The interior

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Fig. 48.-Morphology of Medusida. 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; 6 The same viewed from below. The dotted line indicates the margin of the velum.

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

* The forms here figured (figs. 48, 49), though in all respects anatomically identical with the true Medusa, and originally described as such, are now known to be in reality the medusoid buds of fixed Hydroids. They illustrate the structure and form of the Medusa, however, just as well as though they were completely independent in their development.

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.

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 Medusida have now been proved to be really of this nature, and it may fairly be doubted if this will not ultimately be found to apply to all. As to the value, however, of the order Medusida, the present state of our knowledge is well expressed by the following conclusions which have been drawn up by Professor Greene :"1. That several of the organisms formerly described as Medusida are the free gonophores of other orders of Hydrozoa.

"2. That the homology of these free gonophores with those simple expansions of the body-wall which in Hydra and some other genera are known to be reproductive organs by their contents alone, is proved alike by the existence of numerous

transitional forms and by an appeal to the phenomena of their development.

3. That many of the so-called Medusida may, from analogy, be regarded as, in like manner, medusiform gonophores. 4. But that there may exist, nevertheless, a group of Medusid forms, which may give rise by true reproduction to organisms

a

Fig. 49.-Group of "naked-eyed Medusa." a Sarsia gemmifera, with medusoids arising from the sides of the central polypite (after Greene); b Modeeria formosa (after Forbes); c Polyxenia Alderi (after Gosse).

directly resembling their parents, and therefore worthy of being placed in a separate order under the name Medusida."

The same authority concludes by remarking that to the order as above defined "may be referred provisionally that large assemblage of forms anatomically similar to true Meduside, but whose development is unknown." Besides the large group of forms thus temporarily admitted, all the Trachynemide and Eginida are stated by Gegenbaur to fulfil the conditions of the above definition, and should therefore be looked upon as true Medusida. The Geryoniidæ, so far as known at present, are also developed directly from the ovum, and must therefore find a place here.

As to the development of these true Medusida, little is known for certain. It appears, however, that in Trachynema, Eginopsis, 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 Hydrozoön by which the medusoid was produced. It has also been shown that various of the true Medusa (Cunina, Ægineta, &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 eproduction. Technically these are called "tritozoöids," as being derived from organisms which are themselves but the generative zoöids of another being. This singular phenomenon has been observed in various medusiform gonophores (e.g., Sarsia gemmifera, fig. 49, a), 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" Medusa 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.

CHAPTER XI.

LUCERNARIDA, GRAPTOLITIDE, AND

HYDROCORALLINE.

SUB-CLASS IV. LUCERNARIDA (Acalephæ, 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).

6

A large number of forms included in the Lucernarida were described by Edward Forbes under the name of Steganophthal