376 GEOGRAPHICAL DISTRIBUTION. 



The foregoing data shows that the following thermal classes of surface 

 Siphonophores can be distinguished : — 



1. Arctic, with a maximum of about 45°. 



2. Warm-water species, with a minimum of about 50°. 



3. Tropical species, with a minimum of about 65°. 



4. Species with a very wide temperature range, but not truly eurythermal 

 because they do not exist under Arctic conditions; while one of them, 

 at least, is seldom found in temperatures above 70°. 



5. There are a few species which have been taken so seldom that it is impossible 

 to make any statement about them as yet. 



Seasonal Fluctualions in Horizontal Distribution. 



A resume of our rather scant}- knowledge in this field follows naturally 

 after the discussion of temperature. It has long been known that in sunmier 

 certain warm-water Siphonophores often occur far to the north of their winter 

 range in regions within the influence of the Gulf Stream. Examples are afforded 

 by the almost yearly occurrence of Agalma elegans in Narragansett Bay, and the 

 occasional capture of Diphyopsis dispar off the south coast of Newfoundland 

 (Bigelow, : 09b) ; of Physalia in the Bay of Fundy, and of Velella at the Hebrides. 

 Similarity, but more regularly, the Norwegian sea is invaded from the south via the 

 Faroe-Shetland Channel in the middle of the summer by Physophora and Agal- 

 midae ("Cupulita sarsi" Damas, :09, p. 107) which thereafter form a striking 

 constituent of the plankton of the Norwegian Sea. 



The movements of Muggiaea atlantica in the English Channel and the Irish 

 Sea are known in some detail, thanks to Gough (: 05). This species appears off 

 Ushant at the end of April or beginning of May, and extends thence northward 

 and eastward with the advance of the season. By September it has reached the 

 Irish Sea, and has spread around the southwest shores of Ireland; but it never 

 penetrates eastward in the English Channel beyond the Isle of Wight. By 

 January it has once more disappeared from the region under consideration. Its 

 northerly spread follows, but lags behind, the rising temperature of the surface 

 waters. Thus it was noticed earliest in a temperature of between 50° and 52°. 

 But whereas Muggiaea is not certainly known from north of Ireland, the 

 isotherm of 52° includes the whole western coast of Great Britain, and the south- 

 ern part of the Norwegian Sea by September; and by Januarj^ it has once again 

 receded until it meets the swarm of Muggiaea. This discrepancy suggests that 

 the fluctuation in range of this species is caused less by dispersal through currents 



