HVDROIDA 



45 



placed in a whorl. In fact, between this condition of things and a solitary gonophore seated termin- 

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

 essential enough to be turned to account as fundamentum divisionis. Also Oorhiza has, accordingly, 

 to be included as a synonym under Hydractinia. 



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 Hydractinia and Podocoryne, 

 Hi neks (1868) 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 Hydractinia 

 earned, which are in our Norwegian seas very commonly observed on living specimens of Nassa reticu- 

 lata, in most cases show this open mesh-work of stolons and, therefore, easily run the risk of being 

 undiscerningly characterized as Stylactis. In the African species, Hydractinia Michaelseni Broch 

 and Hydractinia fallax Broch, large colonies show a mixture of characters of Stylactis and of Hy- 

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

 communicate with each other by intermediate forms. It is probable that several species of Stylactis 

 have been based on young colonies of typical species of Hydractinia. This probability, indeed, cannot 

 be contested even by 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 Jaderholrn (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 Sarsii (Steenstrup) Bonnevie. 



1846 Podocoryna carnea, M. Sars, Fauna littoralis Norvegise, Heft i, p. 7. 



1850 Podocoryne Sarsii, Steenstrup, in: Lutken, Nogle Bemserkninger om Medusernes systematiske 



Inddeling, p. 33. 



1872 Stylactis Allman, Monograph of the Gymnoblastic or Tubularian Hydroids, p. 303. 



1892 Podocoryne carnea, L,evinsen, Meduser, Ctenophorer og Hydroider fra Gronlands Vestkyst, p. n. 

 1899 P ars > Ssemundsson, Bidrag til Kundskaben om de islandske Hydroider, p. 50. 



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



The reptant 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 abruptly cut off. The polyps attain a length of 2.5 mm. 

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

 zooids have not yet been pointed out. 



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

 developed polyps of the same size as the sterile nourishing individuals. 



Material : 



Greenland, the harbour of Godthaab . . . depth 12 fath. (on Hyas arancus}. 

 Iceland, Seydisfjord ( on Carcinus maenas). 



