sion that the genus was unequivocally referable to the Hydrozoa. A similar conclusion has recently been reached by Mr Moseley, who had the opportunity of examining the living animal minutely. According to this observer, the colony of Millepora consists of two kinds of zoöids. The larger zooids Occupy the larger tubes, and have a mouth and from four to six knobbed tentacles: the smaller zoöids inhabit the smaller tubes, have no mouth, and have from five to twenty tentacles. The body-cavities of all the zoöids are united together by numerous cœnosarcal canals, which ramify in the spongy skeleton. On the other hand, Professor Martin Duncan and Major-General Nelson describe the polypes of Millepora as having four, eight, twelve, or more tentacles, arranged in successive whorls, and they regard its structure as decidedly Alcyonarian, the animal being thus truly referable to the Actinozoa. As all the compound Alcyonaria possess cœnosarcal tubes uniting the different zoöids, and as the reproductive organs of Millepora have not yet been described, the proper systematic position of the genus is still open to question.

No such doubt, however, seems to envelop the extraordinary animals known as the Stylasterida, which have always been regarded as true Corals. The skeleton in these forms is a branched calcareous structure, closely resembling in many respects some undoubted corals (such as Oculina). Mr Moseley has, however, recently examined a number of the Stylasteride in the fresh condition, and has determined that they are true Hydrozoa. The colony consists of two kinds of zoöids, large and small. The larger or "alimentary" zoöids are comparatively few in number, with a variable number of tentacles, or sometimes non-tentaculate, with a mouth, and having a special layer of digestive cells lining the body-cavity. The smaller or 66 tentaculate zoöids" are much the most numerous, and are not only like tentacles in shape, but are often placed in circles round the larger zooids, as if they belonged to them. They do not possess tentacles themselves, they have no mouths, and they are devoid of a layer of digestive cells lining the body-cavity, their function being apparently solely prehensile. The cavities of the zoöids are placed in communication with one another by a complex system of canals ramifying in the cœnosarcal skeleton; and the true Hydrozoal character of these coral-like forms is shown by the fact that the reproductive organs are situated outside the bodies of the ordinary zoöids, being in the form of fixed sporosacs developed within sac-like cavities (" ampullæ ") in the skeleton, which at certain periods communicate with the exterior by minute pores.

CHAPTER XII.

DISTRIBUTION OF THE HYDROZOA.

I. DISTRIBUTION OF HYDROZOA IN SPACE.-The genera of Hydrozoa have a wide distribution, the mode of reproduction amongst the fixed forms being such as to insure their extension over considerable areas. The various species of Hydra are of common occurrence in the fresh waters of various regions of the world. Cordylophora, the sole remaining fresh-water genus, has not been found to occur out of the north temperate zone. All the other Hydrozoa, without a known exception, are marine in their habits. The fixed forms-viz., the Corynida, Sertularida, and Campanularida-are represented more or less abundantly in almost all seas, extending from the littoral zone to considerable depths. The oceanic Hydrozoa (Calycophorida and Physophorida) are chiefly characteristic of tropical seas; but they are found also in the Mediterranean, and even in seas not far from, or even within, the arctic circle. Of the Hydrocorallina, Millepora is found in shallow water in the coral-reefs of the West Indies and Pacific Ocean, and the Stylasterids mostly inhabit the warmer oceans, principally at considerable depths.

II. DISTRIBUTION OF HYDROZOA IN TIME.-The fine-grained lithographic slates of Solenhofen and Eichstadt have yielded impressions of Medusa belonging to the existing families of the Æquoride and Trachynemide; and the Lucernarida are represented by an ancient form of the Rhizostomida, in the same formation. With these exceptions, however, there are few fossil remains which would universally be conceded to be of a Hydrozoal nature. The Oldhamia of the Cambrian rocks of Ireland has, indeed, been regarded as belonging to the Hydrozoa; but it is believed by Mr Salter to be really a plant. It consists of a main stem with numerous secondary branches, springing from the axis in an umbellate manner, but exhibiting no traces of hydrothecæ.

The occurrence of Corynida in a fossil condition, except in a few cases, can hardly be said to be free from doubt. Remains possibly referable to this order have been, however, recently discovered in the Paleozoic rocks. The oldest of these was described by the author some years ago from the Lower Silurian rocks of Dumfriesshire under the name of Corynoides. More lately a supposed Corynid called Palaeocoryne has been described from the Carboniferous rocks of Scotland. Species of Hydractinia have also been described from the Cretaceous, Miocene, and Pliocene deposits. The Sertularida and Campanulurida are not certainly known to occur in a fossil condition. The fossils called Dendrograptus, Callograptus, Ptilograptus, and Dictyonema, all at present placed amongst the Graptolites, are, however, not improbably truly referable to the Sertularida.

There can be little doubt but that the large and singular family of the

Graptolitida should really be looked upon as extinct Hydrozoa, though good authorities still place them amongst the Polyzoa. As regards their

Fig. 57.-Didymograptus V-fractus.

distribution two facts are chiefly noticeable. In the first place, no Graptolite, except the doubtful genus Dictyonema, has hitherto been found to occur above the Silurian rocks. The Graptolites may therefore be regarded as characteristic fossils of the Silurian period. Secondly, the diprionidian Graptolites, or those with a row of cellules on each side (e.g., Diplo graptus and Climacograptus), have in Bohemia alone been certainly shown to occur above the horizon of the Lower Silurian rocks. common genus Didymograptus (comprising the "twin" Graptolites, fig. 57) is still more characteristic of the Lower Silurian period. In Didymograptus the polypary consists of two lateral symmetrical branches, with cellules on one side only, springing from a central point or base, which is usually marked by a little spine or radicle."

The

The Hydrocoralline are not known to be represented in deposits older than the Tertiary.

CHAPTER XIII.

ACTINOZOA.

I. GENERAL CHARACTERS OF THE ACTINOZOA. 2. CHARACTERS OF THE ZOANTHARIA. 3. ZOANTHARIA MALACODERMATA. 4. ZOANTHARIA SCLEROBASICA. 5. ZOANTHARIA SCLERODERMATA.

CLASS II. ACTINOZOA.-The Actinozoa are defined as Cælenterata with a differentiated digestive sac opening below into the somatic cavity, but separated from the body-walls by an intervening "perivisceral space," which is divided into a series of compartments by vertical partitions or "mesenteries," to the faces of which the reproductive organs are attached.

The Actinozoa (fig. 59), therefore, differ fundamentally from the Hydrozoa in this, that whereas in the latter the digestive cavity is identical with the somatic cavity, in the former there is a distinct digestive sac, which opens, indeed, into the somatic cavity, but is, nevertheless, separated from it by an intervening perivisceral space. As a result of this, the body of a typical Actinozoön (fig. 58) exhibits on transverse section two concentric tubes, one formed by the digestive sac, the other by

the parietes of the body; whereas the transverse section of a Hydrozoon exhibits but a single tube, formed by the walls of the combined digestive and somatic cavity.

Histologically, the tissues of the Actinozoa are essentially the same as those of the Hydrozoa, consisting of the two fundamental layers, the " ectoderm " and the "endoderm." In the Actinozoa, however, there is a much greater tendency to a differentiation of these into specialised structures, and in some members of the class muscular fibres are well developed. Thus, the Sea-anemones have a well-developed series of longitudinal and circular muscular fibres, of which the former

Fig. 58.-A, Transverse section of a Hydrozoon, showing the body-cavity in the form of a single tube enclosed by the body-walls. B, Transverse section of an Actinozoon; s Digestive sac; One of the primary mesenteries, dividing the body-cavity into vertical compartments. Between the six primary mesenteries are seen the secondary and tertiary mesenteries, which fall short of the walls of the stomach. a Ectoderm; ¿Endoderm.

become radial in the disc and base. Cilia are often present, especially in the interior of the somatic cavity, where they serve to promote a circulation of the digestive fluids contained therein. The sole digestive apparatus in the Actinozoa consists of a tubular stomach-sac, which communicates freely with the outer world by means of the mouth, and opens inferiorly directly into the general body-cavity. In most, the "perivisceral space" between the body-walls and the digestive sac is subdivided into compartments by a series of vertical lamellæ, which are called the "mesenteries" (fig. 58, m). Upon the faces of these are borne the reproductive organs in the form of band-like ovaria or spermaria. There are no differentiated respiratory organs as a rule. Some forms, however, which live half-buried in sand or mud, have lobed and crimped organs attached to or near the tentacles, which have been supposed to act as breathing-organs; whilst structures supposed

to be gills are developed in some Zoanthids on either side of the primary mesenteries.

Thread-cells, often of very complicated structure, are almost universally present, some few forms having been asserted to be

Fig. 59.-A, Actinia mesembryanthemum, one of the Sea-anemones (after Johnston); B, section of the same, showing the mouth (a), the stomach (6), and the bodycavity (c).

without them; and some of the Actinozoa are able to sting very severely.

A nervous system has not yet been proved to exist in the Actinozoa generally, except in the Ctenophora, and in none are there any traces of a vascular system. Some Actinia are said to have short optic nerves distributed to the pigment-masses at the bases of the tentacles, and these masses are clearly of a sensory nature; whilst the same animals are affirmed to have a general nervous system as well.

Distinct reproductive organs occur in all the Actinozoa, but these are internal, and are never in the form of external processes as in the Hydrozoa. Sexual reproduction occurs in all the members of the class, but in many forms gemmation or fission constitutes an equally common mode of increase. Some Actinozoa, therefore, such as the common Sea-anemones, are simple organisms; whilst others, such as the reef-building corals, are composite, the act of gemmation or fission giving rise to colonies composed of numerous zoöids united by a cœnosarc. In these cases the separate zoöids are termed "polypes," the term "polypite " being restricted to the Hydrozoa. In the simple Actinozoa, however, the term