BIGELOW: EXPLORATION'S OF THE COAST WATERS. 



241 



Aside from corroborating our earlier work, the plankton records 

 for 1915 are chiefly valuable for the light they throw on tlie immigra- 

 tion of northern organisms into the Gulf of Maine, and on the seasonal 

 changes in the relative faunal importance of the various boreal or- 

 ganisms there. 



Previous to 1915, we had never found any unmistakable Arctic 

 component in the plankton, for though three species common in Arctic 

 waters, Calanus hypcrhorcus, Clione limacina, and FAihrohnia hamata 



■^'^ 



/ / 



Fig. so. — Localities (•) in the Gulf of Maine where tropical and subtropical plank- 

 ton organism have been taken, 1912-1915. 

 H = several records. 



have been taken in some numbers, the first two may be endemic 

 (p. 302) , while the last is as likely to have reached the Gulf from the 

 mid-layers offshore, as from the north (1914a, 1915). And while a 

 few x\rctic organisms have been recorded in the past, e. g., the medusa 

 Ptychogena and the ctenophore Mertensia, (Agassiz, 1865) it is not 

 possible to connect their occurrence with hydrography. Conse- 

 quently the discovery of a plankton element of unmistakable northern 



