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 Cory-nida, Sertularida, and Campanularida - are represented more or less abundantly in almost all seas, extending from the littoral zone to considerable depths. The oceanic Hydivzoa (Calyco-phoridae and Physophoridae) 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 Hydrowallinae, Millepora is found in shallow water in the coral-reefs of the West Indies and Pacific Ocean, and the Stylasterids are almost cosmopolitan, the species ranging from the neighbourhood of the coast-line to great depths in the ocean.
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 AEquoridae and Trachyne-midae; and the Lucernarida are represented by an ancient form of the Rhizostomidae 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 Oldkamia 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 hydrothecae.
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 Palaeozoic 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 Sertidarida and Campanularida are not certainly known to occur in a fossil condition. The fossils called Dendrograptns, Callograptus, Ptilo-grap/us, 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 Graptolitidae should really be looked upon as extinct Hydrozoa, though good authorities still place them amongst the Polyzoa. As regards their 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., Diplograptus and Climacograptus), have in Bohemia alone been certainly shown to occur above the horizon of the Lower Silurian rocks. The common genus Didy-mograptus (comprising the "twin" Graptolites, fig. 67), is still more char acterislic 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 "radicie."
Fig. 67. - Didymograptus V-fractus.
The Hydrocorallinae. with the exception of some Cretaceous forms allied to Millepora, are not known to be represented in deposits older than the Tertiary.