HYDROIDA II 



129 



The principal difference, as compared with Sertularia cuprcssina, is apparent, even in quite 

 young colonies, from the shape of the hydrothecae. Sertularia 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 Abietinaria 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 gonothecte in Sertularia 



2 00 m. boom. 1 000 m. ... 2 000 m. 



Fig. LXVIII. The distribution of Sertularia ttnera 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 cuprcssina. 



Jaderholm (1909 p. 94) suggests the possibility that Dynamena unilateralis 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 tenera 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 mere accident. Ritchie (1911 p. 218) believes the species to 



The Ingolf-Kxpedilion. V. 7. *■ i 



