Ι⅛e Hydrozoa correspond to the Linnaean genera Hydra, Tubularia, Sertularia, and Medusa. The name was applied by Huxley in 1856 to a group corresponding to that termed Hydromedusoe by Vogt (1851) and Medusoe by Leuckart (1853), and embracing the forms placed by Gegenbaur in his Elements of Comparative Anatomy (1878) in four classes, viz., Hydromedusoe, Calycozoa, Thecomedusoe, and Medusoe. Our knowledge of the structure and life-history of the Hydrozoa, many of which, on account of their delicacy and oceanic habits, are excessively difficult to obtain in a state fit for investigation, has greatly extended within the last five years. Whilst in the two decades preceding this period the admirable researches of Huxley, Gegenbaur, Agassiz, and Allman had brought to light and systematized a vast mass of information with regard to these organisms, the later observations of Claus, the Hertwigs, Haeckel, and Metscħnikoff, have corrected, extended, and added to their history, especially in respect of embryological and histological detail. An epitome of the present condition of our knowledge of the group is afforded by the subjoined tabular classification of its families, orders, and sub-classes.

The definition and synonymy of the divisions recognized will be entered into, after a sketch has been given of the common structural features of typical Hydrozoa. [table]

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The Hydrozoa present a greater simplicity of ultimate structure than do any animal organisms possessed of as great a complexity of external form. As in all Metazoa or Enterozoa, the life cycle of a hydrozoon starts with an egg which is at first a single cell or unit of protoplasm, but proceeds after fertilization to multiply by transverse fission in such a way that the resulting cells or units are arranged in two layers, each one cell deep, disposed around a central cavity—the enteron or archenteron. The sac thus formed is known as a diblastula (figs. 1, 2, and 25). By the formation of a mouth to the sac, the enteron acquires the functions of a digestive retort in which food matters taken in at the mouth are brought into a chemical condition suitable for the nutrition of the surrounding cells. The two layers of cells (of which the outer only acquires additional layers by the division of the primary cells, and that by no means in all cases) received from Allman (Phü. Trans., 1855) the names respectively of the ectoderm and the endoderm, having previously been shown by Huxley (1849) to be the fundamental membranous constituents of which the most varied parts of the more complex Hydrozoa— such as tentacles, swimming bells, and air-bladders— are built up in the adult condition. Huxley also pointed out the identity of these membranes with the two primary layers of the vertebrate embryo. The endoderm and the ectoderm, which present themselves, as is now known, in the diblastula (or gastrula) phase of all Enterozoa, remain in Hydrozoa (and also in the allied groups of Coeleηtera) as permanently distinguishable elements of structure. This important disposition is associated with and dependent on the simple character which the archenteron or primitive digestive space retains. Into whatever lobes or processes the sac-like body may be, so to speak, moulded, whether tentacles or broader expansions, into these the cavity of the archenteron is extended in the first instance; and where the actual cavity is obliterated the endodermic cell-layer remains to represent it (Gefässplatte or endoderm-lamella, see figs. 7 and 16).

Conversely, whatever canals or spaces are discovered in the substance of a hydrozoon (excepting only the cavity of ectodermal otocysts) are simple and direct continuations of the one original enteric cavity of the diblastula, and all such spaces are permanently in free communication with one another.

The whole of the Hydrozoa seem to present a lower grade of structure than the Actinozoa, in so far as the latter, whilst retaining permanently free communication between all parts of the archenteric space, yet exhibit a differentiation of this space into an axial and a periaxial portion—a digestive tube and a body cavity. The differentiation has only to proceed a step further, namely, to the closure or shutting off of the axial from the periaxial portion of the archenteric space, and we obtain the condition which characterizes the adult forms of the Coelomata, or animals with blood-lymph space distinct from digestive canal. With the attainment of the coelomate condition, the two fundamental cell-layers, ectoderm and endoderm, which still appear in the embryo, become so far interwoven, and their products so highly differentiated, that it is no longer possible to recognize them as anatomical structures in the adult.

The only deep-seated distinction between Hydrozoa and Anthozoa (the Actinozoa being thus termed when the Cteηophora are detached from them) appears to be the particular differentiation of the archenteric space va Anthozoa which has just been noted. It is no longer possible to •separate the two groups from one another as Exoarii and Endoarii, as was proposed by Rapp (Ueber die Polypen im Allgemeinen und die Actinieη insbesondere, Weimar, 1&29) —the first term indicating the Hydrozoa as possessed of •external generative organs, whilst by the latter term the Anthozoa are pointed to as having internal generative organs. This distinction breaks down completely in the case of Lucernaria, and even in that of the so-called phanero-carpous and some other medusae which discharge their genital products by the mouth, and quite rarely by rupture of the outer body-walk The tendency to form calcareous deposits in the deep layers of the ectoderm, or mesoderm, as it has been termed, exhibited almost universally by the Anthozoa (whence the name Coralligena applied to them), is distinctive of them, though it has been shown first by Louis Agassiz, and more fully and recently by Moseley, to be paralleled among Hydrozoa, by the external calcareous deposits of the abundant and widely distributed Millepores and Stylasterids. A minute distinction between Hydrozoa and Anthozoa, which does not, however, hold good universally, is found in the form of the barbed threads ejected by the nematocysts. Instead of the complicated forms present in the latter group, the Hydrozoa are usually provided with either an unbarbed thread or one in which the barbs are confined to three at the base and a few minute barblets (fig. 5).

Fundamental Forms of the Hydrozoa.— The diblastula derived from the egg of a hydrozoon, when provided with •a mouth, may be spoken of (as are the equivalent forms in other animals groups) as a person. Either this person elongates and develops tentacles in a circlet around or near the mouth, and usually becomes fixed by the aboral pole of the sac-like body, or the sac gradually assumes the form of a clapper-bell or of an umbrella with greatly thickened handle, the mouth being placed at the free end of the handle or of the clapper, and the animal freely swimming by the contractions and expansions of the dome of the bell (disc of the umbrella). The two forms of persons are known, —the former as the “hydriform” (2, 3 in fig. 16), the latter as the “medusiform” (4, 5, 6 in fig. 16).

The HYDRIFORM persons usually occur as fixed branching •colonies or trees (figs. 36 and 37) produced by lateral budding from an original hydra-form developed from a diblastula.

The hydriform person in its most fully developed state is seen in the colonies of Tubularia. In such a colony a number of hydriform persons are united like the flowers of ⅛ plant on its branches (whence Allman’s terms hydranth, hydrophyton). Each hydriform person (fig. 35) has an elongated body with oral and aboral pole. The mouth is placed centrally at the oral pole, which is somewhat enlarged and conical At the apex of the cone, immediately around the mouth, is a circlet of small tentacles ; at the base of the cone is a second circlet of larger tentacles', the surface -of the oral cone is termed the hypostome. In other genera {e.g., Hydra, fig. 42) the smaller circle of tentacles is wanting; in others, again, the tentacles are irregularly placed and not concentrated into one circlet (fig. 38). We regard the former as the typical condition. In the hydriform persons of the Scyphomedusoe (figs. 26 and 27) the vertical axis is much shortened, the hypostome is flat, and the whole body cup-like or hemispherical.

The tentacles of the hydriform person are sometimes hollow (Hydra, Gaτveia nutans, Hydrocorallinoe), being mere prolongations of the sac-like body; but usually, though the endodermal cell-layer is continued into them, they are solid (2 in fig. 16). Very generally the tentacles of the hydra-form are indefinite in number, but in those belonging to the group of Scyphomedusoe a primary series indicating four radii (perradial) can be distinguished, to which are added four intermediate to these, marking four secondary radii (interradial), whilst eight more placed between the eight of the perradial and interradial series are known as adradial tentacles. The surface of the hydraform may be entirely naked, or encased in a horny tube (perisarc) formed by the ectoderm: this may be confined to the aboral portion of the hydranth and to the common stem which unites the persons of a colony, or it may rise up and form a cup (or hydrotheca) around the oral region of the hydranth (figs. 32 and 33).

The bodies of all hydriform persons, as well as the tentacles, are excessively contractile, and when hydrothecae are present can be withdrawn into them.

The ectoderm or outer cell-layer furnishes the protective and contractile tissues of the hydra-form. Very usually it is not more than one or two cells deep, and is separated from the endoderm by a structureless lamella of firm consistence. In. Hydra large cells of the ectoderm (neuro-muscular cells of Kleinenberg) bound the external surface (fig. 3) and give off horizontal muscular processes which lie side by side on the structureless lamella— forming thus a deep muscular coat, the fibrous elements of which are not independent cells. In larger species some of the fibres may become separated from the tegumentary or superficial cells, and acquire the character of independent nucleated corpuscles (Hydractinia, Van Beneden). No nervous elements nor sense-organs occur in any hydra-form (except perhaps the Lucernamoe). In Antennularia some ectoderm cells are amoebiform, and project processes which change shape (nematophυrs). Tactile hairs (palpocils), however, occur on the ectodermal cells, and the solid tentacles are essentially tactile organs. Placed in and between the large cells of the ectoderm (Hydra, Cordylophora, Allman, Kleinenberg, F. E. Schulze) are small nucleated cells which become converted into vesicles containing a three-barbed (figs. 4 and 5) or simple filament (nematocysts). These are frequently grouped on the surface in wart-like processes or “batteries.” Nematocysts also are found in the endoderm; but it is probable that their presence there is due to their having been swallowed.

The endoderm is usually but one cell deep, and lines the entire cavity of the body starting from the margin of the mouth. In the region of the body proper, and in hollow tentacles, the cells are ciliated (fig. 4). In this region they are concerned in the secretion of digestive fluids and in absorption, and sometimes contain coloured granules (hepatic?). Allman found in Myriothela (Phil. Trans., 1875) that the endoderm cells project processes like the pseudopodia of Protozoa, and suggests that solid food particles are incepted by them. T. J. Parker has published similar observation on Hydra (1880). In the solid tentacles the endodermal cells are greatly modified, forming a kind of skeletal tissue, each cell recalling by its vacuolation and firm cell-wall the characters of vegetable parenchyma (fig. 6). In the stems of Siphoηophora endoderm cells give origin to muscular processes like those of the ectoderm (Claus). This latter fact has a morphological significance which cannot be too gravely estimated.

Generative products are not developed by any hydriform persons (excepting the Lucernaria?) , the sexual process being carried on by a distinct set of buds developed on the sides of hydriform persons. These buds either become medusiform persons, or are degenerated representatives of such persons (sporosacs) (figs. 17 and 18). Even the fresh-water Hydra (fig. 42) does not appear to be an exception to this generalization. The single egg cell of Hydra projects at the breeding season in an ectodermal covering, as a wart, from the lower part of the body. A conical eminence or two nearer the mouth contains the spermatozoa. Each ovarium and each spermarium represents an aborted generative person. According to Kleinenberg the egg-cell and the sperm-cells are both derived from the ectoderm. The Lucernarioe develop internal generative organs (fig. 19) which correspond closely with those of the medusiform persons of the group Scyphomedusoe (see below), with which they are classified. Both ova and testis are endodermal in origin in Lucernaria and in the medusiform persons of the Scyphomedusoe, whilst they appear to be ectodermal in origin in the complete medusiform persons of Hydro- medusoe, though in the degenerate medusiform persons known as sporosacs they may either or both have an endodermal origin.

Medusiform persons usually present themselves as isolated free-swimming individuals, but like hydriform persons they have the power of producing new persons by budding (figs. 44, 45, and 46), which may become detached or may remain connected with the primary person (fig. 57) to form a freely swimming colony (Siphonophora) comparable to the fixed colonies of hydriform persons. Medusiform persons are often produced as the immediate result of the development of the diblastula without any intermediate hydriform phase (Pelagia among Scyphomedusoe, Tracho- medusoe, Narcomedusoe, and probably some A nthomedusoe and Leptomedusoe), but quite as frequently originate as lateral buds upon the body-walls of hydriform persons (figs. 34, 37, and 43), or of other medusiform persons (see below), or as metameric fission-products of hydra-forms. The typical medusa-form is ,a hemispherical cup (the nectocalyx, or umbrella, or disc), from the centre of which rises up a cylindrical or conical process (the manubrium, erroneously ρolypite) at the summit of which is the mouth (4, 5 in fig. 16). Four perradial (see above for use of this term) tentacle-like lobes very commonly surround the mouth, or numerous small tentacles (fig. 58), whilst the margin of the disc is beset with tentacles four in number, or a multiple of four (sometimes six, or one only, or indefinite). The aboral pole is dome-like, and is never attached except in those forms which take their origin as buds on a hydriform colony when the connexion exists at this point. The tentacles are, as in the hydriform persons, some solid, some hollow; both occur in the same individual.

The body is not so completely hollowed out as in the hydriform persons. The mouth leads into a straight tube (the stomach) which occupies the axis of the manubrium, and expands at its insertion into the disc. The disc, even when thick and fleshy, is not fully excavated by the enteric cavity. In young forms the cavity does occupy it right up to the margin, but gradually the lumen disappears (fig. 29), leaving a series of canals and a continuous plate of endoderm (fig. 7) formed by the coalesced walls of the space (the endoderm-lamella of the Hertwigs, see Organismus der Medusen, 1878; the vascular-lamella of Claus, “Polypen und Quallen der Adria,” Wiener Denksch., 1878). The peripheral portion of the lumen of the original enteric cavity forms the ring-canal, which runs all round the margin of the disc, and is continued into the hollow tentacles. The lumen is further retained at intervals in the form of radiating canals connecting the axial enteric cavity with the ringcanal. These may be perradial, interradial, and adradial (see above as to tentacles of hydra-form), and may branch dichotomously in the disc or form networks.

The medusae are thicker and more fleshy to the touch than are the hydra-forms, and are at the same time transparent. This is entirely due to the enormous development of a structureless substance between ectoderm and endoderm,, corresponding to the “Stutz-lamella” or structureless lamella of the hydra-forms. (See figs. 49 and 51, representing sections of Carmarina and of Cunina.) The remarkable development of this substance in a hyaline condition has led to the description of canals and spaces where none exist—the supposed spaces being really occupied by this hyaline substance. F. E. Schulze’s statements as to extra-enteric spaces in Sarsia are thus explained—and more decidedly the supposed circular and longitudinal canals attributed by some authors to the scyphistoma phase of Discomedusoe. In the same manner (according to Claus) Allman’s observations on Siephanoscyphus are reconciled with those of F. E. Schulze on Spongicola— clearly the same form. Siephanoscyphus is devoid of either circular or longitudinal canals, and though it has four remarkable ridges on the enteric wall like those of the scyphistoma of Scyphomedusce (see fig. 26) stands in all probability very close indeed to the Tubularian genus, Berigonirnus.

In a large number of medusa-forms the hyaline gelatinous substance is structureless, but in many of the larger Scy- phomedusoe it is occupied by in-wanderingamoeboid cells derived from the endoderm and by fibrous trabeculae (fig. 8).

The wandering endodermal cells are nutrient in function, and represent so far isolated elements of the enteric canal system.

The medusiform person is fundamentally adapted to swimming movements. The muscular fibres are mostly transversely striated, and are as a rule outgrowths of super ficial ectoderm cells as in Hydra (fig. 9), (though in some cases distinct cells); they are confined to a sheet spread on the oral face only of the disc or swimming-bell (sometimes called sub-umbrella), to the extensile manubrium and tentacles, and to an inwardly directed flap of the margin of the disc known as the velum ( Ve in 4 of fig. 16), which is present in those medusae that are not flattened but conical (bell-like). The muscular fibres on the oral face of the disc and on the velum have a circular direction, interrupted in some cases by radial tracts. The direction of the swimming movements is obvious from this arrangement.

. The velum is not a constant element in the medusa’s disc; it serves to contract the space by which water is expelled from beneath the bell in the act of swimming.

All fully-developed Hydromedusoe possess the velum, but only a few of the Scyphomedusa? (Charybdcea), In the former the endoderm plate (vascular lamella) is not continued into it; in the latter vessels of the enteric system are present in it (fig. 21), and, being probably morphologically distinct, it has been here termed the “pseudo-velum.”

Unlike the hydra-forms, the medusa-forms of Hydrozoa possess in addition to the tentacles highly-developed senseorgans and ganglionic nerve-centres and nerves. The senseorgans appear to be either eye-spots, or else otocysts, or to combine the functions of both. In addition to these are olfactory tracts or pits connected with the preceding. The sense-organs are placed along the margin of the disc (hence called marginal bodies), and are of three kinds:— (1) ocelli—rounded pigment spots, rarely provided with a lens {Lizzia) (fig. 10), always placed at the base of a tentacle or in the radius of one on the oral surface {Lizzia), entirely ectodermal in origin; (2) vesiculi or otocysts—formed (as discovered by the Hertwigs, 1878) by an invagination of the ectoderm (fig. 11) containing concretions and hair cells; either open or entirely closed, generally numerous, and placed between tentacles, sometimes at the bases of tentacles ∖ θbelia) ; (3) tentaculocysts—which are reduced and modified tentacles; into them alone of the three kinds of mar ginal bodies do the endoderm and, in the more complex, the enteric canal system enter (figs. 12, 13, and 30). The endodermal sac forms the axis of the tentaculocyst, its cells secrete crystalline concretions, and it functions as an otocyst; pigment spots, which may have cornea, lens, and retina well developed, are formed sometimes to the number of six (Charybdoea) on the ectoderm of the tentaculocyst (fig. 13). The olfactory sense-epithelium (fig. 14) is either distributed in a continuous band on the margin of the disc (Hydromedusoe, discovered here by the Hertwigs), or it is confined to deep pits (foveae nervosae) from each of which a tentaculocyst arises (discovered in the Scyphomedusae independently by Schäfer and Claus). With some exceptions, medusae provided with ocelli are destitute of vesiculi, which alone occur in the vesiculate Leptomedusoe. Tentaculocysts characterize to the exclusion...