. The Danish Ingolf-Expedition. Scientific expeditions; Arctic Ocean. 36 HYDROIDA II creeping stolons. This thus confirms, as Kramp points out, the doubts which have been expressed by certain previous writers as to the fundamental importance of the structure of the colony for hydroid s\ stematics, and partly effaces the limit between creeping species and species with upright, composite colonies. The genus Halecium is characterised by the lively renovation of its hydranths. This should, it would seem, be explained as due partly to the lack of special defensive individuals, partly to the minimal
. The Danish Ingolf-Expedition. Scientific expeditions; Arctic Ocean. 36 HYDROIDA II creeping stolons. This thus confirms, as Kramp points out, the doubts which have been expressed by certain previous writers as to the fundamental importance of the structure of the colony for hydroid s\ stematics, and partly effaces the limit between creeping species and species with upright, composite colonies. The genus Halecium is characterised by the lively renovation of its hydranths. This should, it would seem, be explained as due partly to the lack of special defensive individuals, partly to the minimal size of the hydrotheeae, and finally also to the large dimensions of the polyps, which probably render them particularly attractive to creatures preying upon hydroids generally. The marked reno- vation activity often leads to the formation of whole piles of hydrothecae, or apparently small branches, for which Schydlowski (1901) has introduced the term "psel|dohydrocauli". Not infrequently also, heteromorphotic renovates may be observed; I have previously (1909 p. 151) noticed that the formation of gonothecae as heterorenovates is characteristic of the male in Hale- cium ornatum Nutting. Heteromorphotic renovates of a more accidental nature will be noted later on in several species; these are of great interest, since renovates of this kind have, according to Hadzi (1915) only been met with in nature among Halecium and Symtheciidir. The question has, however, been too little studied as yet to permit of our drawing further conclusions from this, but the point should be kept in mind, until it has been fully cleared up. Halecium halecinum (Linne) Oken. 1758 Sertularia halecina, Linne, Systema Naturae, Ed. 10, p. 809. 1815 Halecium halecinum, Oken, Lehrbuch der Naturgeschichte, vol. 1, p. 91. Stiffly built, as a rule doubly pinnate colonies with polysiphonic main stem. The branches are as a rule pinnately ramified in the same plane as the main stem, or the colonies may, mo
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