. The Annals and magazine of natural history; zoology, botany, and geology. Natural history; Zoology; Botany; Geology. 326 M. C. Mereschkowsky on the Hydroida. slender hydrantlis, all unmistakably recall a Hydractinia or a Podocoryne ; so that from tlie trophosome alone it is impossible to distinguish Oorhiza from one of the above-mentioned genera. But the sexual individuals (gonosomes) present such charac- teristic peculiarities that it is impossible to place the Hydroid in question in any of the known genera of Hydractiniidse. Podocoryne^ as is well known, has the gonosomes consisting of bla
. The Annals and magazine of natural history; zoology, botany, and geology. Natural history; Zoology; Botany; Geology. 326 M. C. Mereschkowsky on the Hydroida. slender hydrantlis, all unmistakably recall a Hydractinia or a Podocoryne ; so that from tlie trophosome alone it is impossible to distinguish Oorhiza from one of the above-mentioned genera. But the sexual individuals (gonosomes) present such charac- teristic peculiarities that it is impossible to place the Hydroid in question in any of the known genera of Hydractiniidse. Podocoryne^ as is well known, has the gonosomes consisting of blastostyles upon which Medusae are produced j Hydractinia differs in that the same blastostyles give origin to sporosacs ; lastly, Oorhiza has no blastostyles at all, but the sporosacs rise from the hydrorhiza itself. It is true that in Podocoryne the presence of the blasto- styles is not a constant character : their absence, indeed, is constant in Podocoryne areolata; in others, as for example Podocoryne aculeata, there may or may not be a blasto- style ; and, finally, in Podocoryne carnea the Medusge are constantly produced upon blastostyles. But this does not apply to the genus Hydractinia^ in which the presence of blastostyles is a perfectly constant character and essential to the genus. The genealogical relations between the genera and species of the family Hydractiniidae may be expressed by the accom- panying diagram :— echinata. liolyclina. Hydractinia Oorhiza boreaUs. P. areolata. P. carnea. / Intermediate form*. Podocoryne * This hypothetical intermediate form must have existed analogically with Podocoryne aculeata ; it must have possessed indifferently sporosacs on blastostyles and without blastostyles: on one side Hydractinia was developed (analogically with Podocoryne carnea); on the other Oorhiza (analogically with P. areolata). If ever this hypothetical form is found, it will be necessary to unite the genera Hydractinia and Oorhiza into a single one, or
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