28 



HYDROIDA II 



Stegopoma plicatile is an arctic species, which can nevertheless penetrate into the boreal region 

 (fig. VIII); its occurrence is, however, somewhat of a mystery. The species must be described as 

 circumpolar, having been recorded from Davis Strait through the Norwegian Sea, the Barents Sea, 

 the Kara Sea, and the Siberian Frozen Sea to the Bering Sea and the Sea of Okhotsk. Bathymetri- 

 cally it is mostly found in the lower part of the littoral region. From this, then, the species should 

 be noted as arctic; as a matter of fact, however, this would not agree with the data from the northern 

 Atlantic regions. There are a couple of isolated finds from the cold area, but the species has not yet 

 been noted either from the east coast of Greenland or from the eastern side of Davis Strait, and 



Fig. VIII. The occurrence of Strgopoma plicatile in the Northern Atlantic. 

 In the hatched parts the literature notes a scattered, although common occurrence. 



also appears to be entirely lacking in Iceland waters. On the other hand, we find that it is very 

 common along the eastern side of Davis Strait, and — according to the records — along the west 

 coast of Norway ; i. e. just in those places where the warmer water layers predominate in the bathy- 

 metrical area of its distribution. It may also be noted, in this connection, that the species occurs on 

 the LophoJiclia reefs in the Troudhjem Fjord, in quantities unknown elsewhere. This occurrence is 

 altogether mysterious in the case of a species otherwise only met with in high arctic waters; one 

 would naturally expect that any extension of its distribution would follow the colder eastern shores, 

 not the boreal western coasts of the continents. It is difficult at present to give any satisfactory 

 explanation of this peculiarity. 



