THE HYDROZOA form one of the three classes into which the Codentera nematophora (distinguished from the Ccelentera porifera, or Sponges) have been divided, - recognized as such in the article C(ELENTERA, to which the reader is referred. It results from observations made by Ernst Haeckel, since that article and the article ACT1NOZOA were penned, that the Ctenophora should not be regarded as a class equivalent to the Ilydrozoa and Actinozoa, nor as a subdivision of the latter class, but that they must be considered as a peculiar modification of the medusiform Ilydrozoa (see final paragraph). If this conclusion be accepted, it will be necessary to divide the Ilydrozoa into two primary groups or grades, for which the names Polypomorpha and Ctenophora are proposed.

The Ilydrozoa correspond to the Linmean genera Hydra, Tubularia, Sertularia, and Medusa. The name was applied by Huxley in 1856 to a group corresponding to that termed Hydromedusce by Vogt (1851) and Medusce by Leuckart (1853), and embracing the forms placed by Gegenbaur in his Elements of Comparative Anatomy (1878) in four classes, viz., Hydromedusx, Calycozoa, Thecomedusa,, and Medusa?. Our knowledge of the structure and life-history of the Ilydrozoa, 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 Metschnikoff, have corrected, extended, and added to (heir 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 Ilydrozoa.

Order 1. LUCERNAR11E. Examples.

Lueernaria (fig. 10).

Fam. 1. Eleutherocarpidse '''''7 ILdielystus.

Order 2. DISCO,IEDrsX, (11aeckel).

Sub-Order 1. Cubostomm.

Fam. 1. Pi otephyridx.

„ 2. Nausithoidse Nausithoe.

J Chrysnora (fig. 24, b) Fam. 1. Pe1sgidm Pelagic.

Sub-Order 8.121lizostomm.

Fam. 1. Tetragsmelhe . ( Cephea. Cassiopeia.

( Ithizomoma (fig. 24, a).

++2. id onogamelia3 Crambessa.

Order 3. CONOMED175/F. (11aeckel).

Fam. 1 Charybdeidac .......... Charybdma (figs. 20-23).

2.13ursarldse.

Order 4. PEROMEDUSA (Ilneckel). Fam. 1. Perlphyllithe.

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 11fetazoa 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 layers2 by the division of the primary cells, and that by no. means in all cases) received from Allman (Phil. 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 llydrozoa - such as tentacles, swimming bells, and air-bladdersare 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 Ccelentera) 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 InhAs nr nrneessns the body may hn_ sn 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 endoclermic cell-layer remains to represent it (Gefiissplatte 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.4 The whole of the Hydrozoa seem to present a lower grade of structure than the Actimozoa, 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 arcbenteric space, and we obtain the condition which characterizes the adult forms of the Ccelomata, or animals with blood-lymph space distinct from digestive canal.' With the attainment of the ecelomate 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 Ilydrozoa and Anthozoa (the Actinozoa being thus termed when the Ctenophora are detached from them) appears to be the particular differentiation of the archenteric space in 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 (Lieber die Polypen ins Allgemeinen nisei die Actinien insbesondere, Weimar, 18L)9) - 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.2 This distinction breaks down completely in the case of Lucernaria, and even in that of the so-called phanerocarpous and some other mednsm which discharge their genital products by the mouth, and quite rarely by rupture of the outer body-wall. 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 Ilydrozoa, by the external calcareous deposits of the abundant and widely distributed Millepores and Stylasterids. A minute distinction between Ilydrozoa 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 Ilydrozoa 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 Ilydrozoa. - 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 a plant on its branches (whence Allman's terms hydranth, hydrophyton). Each hydriform person (fig. 35) has an -elongated bocly 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 810011 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 Scyphomedusce (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, Garveia unions, Hydrocorallince), 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 Scyphomedusce 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 hydra-form may be entirely naked, or encased iii 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 hydrothecm 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. InHYdra 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

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 (hepatict). Allman found in Jfyriothela (Phil. Trans., 1875) that the endo

Generative products are not developed by any hydriform persons (excepting the Lucernarice), 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 Lucernarice develop internal generative organs (fig. 19) which correspond closely with those of the medusiform persons of the group Scyphomedusce (see below), with which they are classified. Both ova and testis are endodermal in origin in Lucernaria and in the medusiform persons of the Scyphomedusce, whilst they appear to be ectodermal in origin in the complete medusiform persons of Ilydromedusce, though in the degenerate medusiform persons known as sporosacs they may either or both have an endodermal