. The Danish Ingolf-Expedition. Scientific expeditions; Arctic Ocean. HVDROIDA II 129 The principal difference, as compared with Scrtularia cupressina, is apparent, even in quite young colonies, from the shape of the hydrothecce. Scrtularia tenera (fig. LXVII) has larger hydro- thecse than the mentioned species, and the highly divergent, quite large distal free part gives the colonies a peculiar appearance, very suggestive of Abictinaria filicula (Ellis et Solander) with which also its dimensions very nearly agree. A character applicable to the larger colonies on the whole is the fact that the
. The Danish Ingolf-Expedition. Scientific expeditions; Arctic Ocean. HVDROIDA II 129 The principal difference, as compared with Scrtularia cupressina, is apparent, even in quite young colonies, from the shape of the hydrothecce. Scrtularia tenera (fig. LXVII) has larger hydro- thecse than the mentioned species, and the highly divergent, quite large distal free part gives the colonies a peculiar appearance, very suggestive of Abictinaria filicula (Ellis et Solander) with which also its dimensions very nearly agree. A character applicable to the larger colonies on the whole is the fact that the main stem in Sertularia tenera does not exhibit any superiority in point of thick- ness over the branches, whereas in Sertularia cupressina it stands out in virtue of its greater dimens- ions; this feature, however, is not always absolutely reliable. — Despite their great variability, the gonothecse also present certain differences as between the two species. The gonothecse in Scrtularia. 20om. _ 600 m. ._. iooo m. 2oooi7i. Fig. LXVIII. The distribution of Sertularia tenera in the Northern Atlantic. In the hatched region a common occurrence is recorded. tenera are, even where the polygonal form is markedly prominent, never furnished with spines, as is normally the case with Sertularia cupressina. Jaderholm (1909 p. 94) suggests the possibility that Dynamena u///'lateralis Bonnevie (1899 p. 78) might be synonymous with Thujaria arctica (Allman). An investigation of the original specimens fully bears out the correctness of this supposition; the specimens are well developed and entirely typical colonies of Sertularia to/era forma spitzbergensis. Sertularia tenera is an arctic-boreal species having its most frequent occurrence at the limit between the two areas; it belongs to the littoral region, and only exceptionally does it move down into the abyssal. The species has never before been met with at such a depth as at the "Ingolf St. 69, and this find must be regarded as a
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