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Identifier: thegeographicalb00benn (find matches)
Title: "The geographical and bathymetrical distribution of the hydroids."
Year: 1921 (1920s)
Authors: Bennitt, Rudolf
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Contributing Library: Boston University Libraries
Digitizing Sponsor: Boston Library Consortium Member Libraries

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to higher latitudesand the cold currents carrying polar animals toward theequator. That this i3 true will readily be seen in thecase of the hydroids by recalling that all hydroids have afree-swimming planula- or medusa-stage, or both, that theirbases of attachment are often carried about by currents,and that even the hydroids alone have been known to liveand reproduce in a free, floating condition. (Fraser,1916;Dendy,1902.) The importance of the medusa-stage in exposing thehydroid3 to the influence of currents is self-evident,especially since practically all the medusa-bearing hydroidslive in shallow water, less than 100 fathoms in depth.Obelia geniculata, perhaps the mo3t cosmopolitan geograph-ically of all the hydroids, doubtless owes much of its dis-tribution to the action of currents on its hordes of medusae.But although this element i3 an important factor in thedistribution of the large numbers of species producing freemedusae, it is not of great importance among the cosmopolitan
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1 \ 27 forma. Of the 27 provinces named in the check-li3t, 15hydroids have been recorded from 12 or more, giving themclaim to be called cosmopolitan. Of these, Obelia geni-culata is the only one which bears medusae. All hydroids have in their life-history a free-swimmingplanula stage, and although little stress has been laid onthi3 as a factor in distribution, I believe that the con-tinuous, steady action of currents on the planulae, eventhough they are free but a short time, has much to do withthe spread of species. The objection will be raised thatthe medusa-bearing forms also have the planula-stage, andyet are not 30 widely distributed. The explanation of thisprobably lies in the fact that medusae are seldom widelydistributed, owing probably to their own inability to resi3tgreat changes. Hence, while hydroids having both medusaeand planulae would be carried as far by currents ag thosehaving only the planula-stage, yet the chances of theirbeing brought into waters where they could cthegeographicalb00benn

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