Archive image from page 142 of The Danish Ingolf-Expedition (1918). The Danish Ingolf-Expedition danishingolfexpe0507ingo Year: 1918 HYDROIDA II 135 of the point. In the northern Sertularia species, we find a general tendency towards spiral arrange- ment of the branches, together with an often very marked approach to unilateral arrangement of the hydrothecse. The spirally placed branches are secondarily ramified, often dichotomieally, but in Sertularia tenera irregularly pinnately. From this then, it is in reality but a short step to Hydrall- mania. And finally, there is a certain amount of
Archive image from page 142 of The Danish Ingolf-Expedition (1918). The Danish Ingolf-Expedition danishingolfexpe0507ingo Year: 1918 HYDROIDA II 135 of the point. In the northern Sertularia species, we find a general tendency towards spiral arrange- ment of the branches, together with an often very marked approach to unilateral arrangement of the hydrothecse. The spirally placed branches are secondarily ramified, often dichotomieally, but in Sertularia tenera irregularly pinnately. From this then, it is in reality but a short step to Hydrall- mania. And finally, there is a certain amount of variation in the Sertularia species, with regard to proportion between the two sinuses, which in some species may be characterised as of equal size. We have thus, as the last and most essential distinctive feature, the fact that the adcauline closing mem- brane in Hydrallmania has a free distal part, whereas in Sertularia, this is reserved for the abcauline membrane. It may be doubtful whether these small differences should be regarded as of generic im- portance; for the present, however, I have found it most correct to follow Levins en in distinguish- ing between the two genera. Hydrallmania falcata (Linne) Hi neks. 1758 Sertularia falcata, Linne, Systema naturae, Ed. 10, p. 810. 1868 Hydrallmania falcata, Hincks, A History of the British Hydroid Zoophytes, p. 273, pi. 58. Colonies with slender, spiral, monosiphonic and segmented main stem, where as a rule each internodium bears a branch. The branches form an open spiral about the stem, and are themselves regularly secondarily ramified pinnately; the main stem has a hydro- theca in the corner at the origin of the branch, or may in rare cases be altogether devoid of hydrothecae. Branches and branchlets are segmented, the irregular internodia bear on the one side (the upper) a more or less close row of hydrothecae, the opening part of which is turned out alternately to either side from the median plane of the branch. On young pi
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