HYDROIDA II 



45 



theca can vary with regard to curvature, and may in extreme cases somewhat resemble Halecium 

 labrosum; in Halecium muricatum however, we always find that the adcauline margin of the hydro- 

 theca is more markedly curved, the abcauline less so. 



Normally, the length of the interuodium is about three times its distal breadth (including the 

 apophyse); we have, however, from the "Ingolf" St 125, a colony which, from its extremely long inter- 

 nodia, must be regarded as a deep-sea variant (fig. XVII b\ forma abyssalis nov. It is not unusual 

 among hydroids to find that the colonies from greater depths exhibit an extension of the single branch 

 parts; in this case, however, the tendency is pushed to an extreme, and at a first glance, the colony 

 presents an altogether alien appearance. It is probably clue to the fact that the species here occurs 

 in the cold area, at the considerable depth of 729 fathoms. The colonies are very slender, about 10 cm 

 high, with polysiphonic stems. The 

 internodia of the minor branches are 

 3—4 mm long, about five times the 

 distal breadth incl. apophyse. The 

 apophyse itself is very distinctly mar- 

 ked and bears a slender hydrotheca. 

 The hydrotheca has close under the 

 diaphragm the typical adcauline thick- 

 ening of the wall, the "pseudodia- 

 phragm" which as a rule is also found 

 in the typical form. The hvdrothecae Fig XVII a _ b 



exhibit the form typical for the spe- Halecium muricatum. 



,, , , , ., a. Internodium and hvdrothecae 



cies; the secondary hvdrothecae are 



from a typical colony from Store 



comparatively short-stalked, but other- H ellefiskebanke, depth 32 fath. 



wise of the same shape as the pri- j. Internodium with hydrothecaa 



mary. The colonies are sterile. From of forma «*?««** from "ingolf 



St. 125. (X 401. * 



the features mentioned it follows that 



the colonies cannot be taken as representatives of a distinct species; they should, however, be distin- 

 guished as the type of a particular forma abyssalis of the original species. 



Halecium muricatum is a circumpolar arctic species, able to penetrate into the boreal region 

 and out to the limits between this and warmer layers (fig. XVIII). Its principal occurrence is restricted 

 to the upper part of the littoral region, but it may, as seen from the foregoing, penetrate far down 

 into the abyssal. It was met with already by the Norwegian North-Atlantic Expedition at a depth of 

 1350 metres. 



Halecium labrosum Alder. 

 1859 Halecium labrosum, Alder, Descriptions of three new species of Sertularian Zoophytes, p. 351, pi. 13. 



Robust, upright, and irregularly branched colonies with polysiphonic main stem. The hvdrothecae 

 are short, symmetrical, with markedly recurvate aperture margin. The basal cavity is very large, and 

 bounded above by the base of the; hydranth; the diaphragm rudimentary or entirely lacking. The 



