HYDROIDA 



45 



placed ill a whorl. In fact, between this condition of things and a solitaiy o-onophore seated teriuin- 

 ally on a polyp wholly reduced to a stalk, there is no great gaj), and at any rate the criterion is not 

 essential enough to be turned to account ;is fundainentuiii divisionis. Also Oorliiza has, accordingh', 

 to be included as a synonym under Ilydractiiiia. 



Stylactis is yet left to be mentioned, being distinguished by the stolons forming an open mesh- 

 work and no continuous chitinous crust. In his excellent elucidation of Hydractiiiia and Podocoryni\ 

 Hi neks (i86S) calls attention to the fact that, in the species then known, the stolons at first form 

 an open mesh-work and do not till later on coalesce into a continuous crust. Young colonies of Ilydractmia 

 canwa, which are in our Norwegian seas very commonly observed on living specimens of N^assa rcficii- 

 lafa, in most cases show this open mesh-work of stolons and, therefore, easih' run the risk of being 

 undiscerningly characterized as Sfy/acfis. In the African species, Hydractinia Michaclscni Broch 

 and Hydracti)ii(i falhix Broch, large colonies show a mi.xture of characters of Stylactis and of I[y- 

 dracti)iia, and, accordingly, there is no reason to maintain the two groups as distinct genera. They 

 communicate with each other b\- intermediate forms. It is i^robable that several species of Stylactis 

 have been based on young colonies of ty[)ical species of Hydractinia. This jsrobability, indeed, cannot 

 be contested even Ijy the occurrence of gonophores, because, in young colonies of Hydractinia carnea, 

 where the stolons have not yet coalesced into a crust, gonophores are frequently observed. A species 

 as Stylactis arctica Jaderholm (1902) has obviously to be judged summarily as such a young Hy- 

 dractinia. Also Stylactis, therefore, has to be included among the synonyms of Hydractinia. 



Hydractinia Sarsi! (Steenstrup) Bonnevie. 



1846 Podocoryiia carnca, M. Sars, Fauna littoralis Norvegice, Heft i, j). 7. 



1850 Podocorync Sarsii. Steenstrup, in : Lvitken, Nogle Beniierkninger om Medusernes systematiske 



Inddeling, p. 33. 



1872 Stylactis — Allman, Monograph of the (yyuinoblastic or Tubularian Hydroids, p. 303. 



1892 Podocorynr carnca, Levinsen, Meduser, Ctenophorer og Hydroider fra Gronlands Vestkyst, p. 11. 

 1899 — — pars, Sa^mundsson, Bidrag til Kundskaben om dc islandske Hydroider, p. 50. 



1899 Hydractinia sarsii, Bonnevie, Norske Nordhavs Expedition, p. 45. 



The rcptant stolons are covered by a continuous chitinous coenosarc, whose surface is studded 

 with small spines, among which are found, often by groups, large, vigorous thorns, up to 0.5 mm. high, 

 of irregular conical shape with closed apex abrupth- cut off. The polyps attain a length of 2.5 mm. 

 and have 10 or 20 tentacles in a dense, proximateh- double, whorl, below the oral portion. Spiral- 

 zooids have not yet been pointed out. 



The gonophores are cryptomedusoid and ]daced, to the number of three or six, round fully 

 developed polyps of the same size as the sterile nourishing indi\iduals. 



Material : 



Greenland, the harbour of Ciodthaab . . . depth 12 fath. (on Hyas aranctis). 

 Iceland, Seydisfjord (on Carcinus viaenas). 



