460 BULLETIN OF THE BUREAU OF FISHERIES 



We have never found Halosphaera dominant in the plankton of the gulf. The 

 richest catches have been over the outer part of the shelf off Nova Scotia (fig. 130; 

 stations 10293 to 10295) and off Mount Desert Island (station 10284) in June, 1915. 

 Most of our records are based on the vegetative stage and on stages in division of 

 the protoplasm (Lemmermann, 1908, p. 21, figs. 71 and 72). Cells with aplanospores 

 have been detected only once in our towings — that is, near Shelburne, Nova Scotia, 

 June 23, 1915 (station 10293), and no attempt has been made to trace the life history 

 of Halosphtera in American waters, as Gran (1902, p. 12) has done so carefully for 

 the Norwegian Sea. 



The seasonal fluctuations of Halosphsera in the Gulf of Maine generally parallel 

 its occurrence in the North Sea, where it is at its maximum in May and its minimum 

 in August. But east of Cape Sable it evidently reaches its greatest abundance later 

 in the season, for Wright (1907) describes it as an important factor in the plankton 

 at Canso, eastern Nova Scotia, in June and July. 75 



It is now well established that Halosphsera is not endemic in the North Sea but 



occurs there only as an immigrant from the Atlantic via the northern route around 



Scotland; and it is primarily of southern — Atlantic — origin in the Norwegian Sea, 



though it may also be endemic there to some degree. Whether it is equally an 



immigrant in the Gulf of Maine is yet to be determined, but the facts that our largest 



catches of it have been made over the outer part of the continental shelf and that 



we have never found it in any great numbers in the inner part of the Gulf point 



in this direction. 



ACANTHARIAN RADIOLARIANS 78 



The swarming of radiolarians, represented by the genus Acanthometron, is 

 a decidedly sporadic event in the Gulf of Maine, as it is in North European waters 

 also (Mielk, 1913), but on such occasions they are extremely conspicuous among 

 the plankton, thanks to their large size, distinctive appearance, and reddish color. 

 Up to the present time we have only once found Acanthometron dominant — that 

 is, on August 22, 1914 (station 10253, fig. 131), when it swarmed off Cape Ann and 

 in the western basin. We have never found Acanthometron before or since in 

 midsummer in the gulf. Apparently it occurs more regularly in early autumn and 

 is more generally distributed then, lor it was comparatively plentiful in the center 

 of the gulf (station 10309), in the northeast corner (station 10316), off Penobscot 

 Bay (station 10318), and off Shelburne, Nova Scotia (station 10313), during the 

 first and second weeks of September in the year 1915. It was a conspicuous ele- 

 ment in the plankton of Massachusetts Bay during the last week of that month 

 (stations 10320 and 10321; fig. 132), but its presence there was short-lived, for none 

 were found a month later (stations 10337, 10338, and 10339, October 26 and 27). 

 Acanthometron has been detected in only one October tow elsewhere in the gulf 

 (a few miles off Penobscot Bay, October 9, 1915, station 10329). It was not found 

 at any of the stations in the western part of the gulf in the late autumn, winter, or 

 spring, but a few specimens were noted on German Bank and in the North Channel 

 on April 15, 1920 (stations 20103 and 20105). 



78 For notes on the temporal occurrence of Halosphaera in the open Atlantic, the Norwegian Sea, and in the Mediterranean 

 see Cleve (1900), Gran (1902), Steuer (1910), and Ostenfeld (1910). 



76 For an excellent account of the northern acantharians see Popofsky, 1905. 



