PLANKTON OF THE GULF OF MAINE 59 



sufficient evidence that its invasion takes place chiefly into the eastern side and from 

 the southwest and south; that is, across the eastern end of Georges Bank and via the 

 Eastern Channel. It is probable (as suggested by Doctor Huntsman in a recent 

 letter) that S. serratodentata also comes to the gulf from the east, drifting with re- 

 current movements of mixed water along the outer edge of the continental shelf oft* 

 Nova Scotia and entering across Browns Bank or through the Eastern Channel, but 

 there is no reason to suppose that any come by way of the Northern Channel or around 

 Cape Sable across the coastal shallows; in fact, it would be very surprising to find any 

 warm-water species journeying along that route. 



Our failure to find S. serratodentata off Cape Cod in autumn, although Septem- 

 ber, October, and November are the months when it is widest spread in the northern 

 parts of the gulf, suggests that the individuals of the species taking part in the 

 successive waves of immigration inward past Nova Scotia seldom survive long enough 

 in the eddy-like circulation of the gulf to journey much beyond Massachusetts Bay 

 in their circuit. The fact that specimens from the outer edge of the continental 

 shelf have been much larger than is usually the case in the Gulf Stream, or in tropical 

 seas generally, corroborates this view, for it indicates a considerable sojourn in the 

 cool band of banks water on the part of S. serratodentata before it enters the Gulf of 



Maine. 



ARCTIC VISITORS 



In the Gulf of Maine the Arctic, like the Tropic, immigrants fall in two categories, 

 depending on whether they are able to survive for a considerable period and even to 

 reproduce to some extent there, or whether they find the high temperature of the 

 water so fatal that they soon perish. The latter group — most typically Arctic — has 

 not been represented within the gidf in our midsummer, autumn, winter, or early 

 springhaulsexceptforanoddMertensia 29 off Penobscot Bay on June 14, 1915 (p. 371), 

 though this ctenophore and the Arctic medusa Ptychogena lactea have previously been 

 recorded in Massachusetts Bay and at Grand Manan in September (A. Agassiz, 1865; 

 Fewkes, 1888) ; but in early May of 1915 both of these cold-water ccelenterates, with 

 the large shelled pteropod Limacina helicina and the appendicularian OiJcopleura 

 vanhoffeni, which are equally characteristic of a northern origin, were taken in the 

 eastern side of the gulf at localities where temperature and salinity gave clearest 

 evidence of an influx of the cold Nova Scotian water past Cape Sable into the gulf at 

 the time (fig. 32). Since each of these species was represented by several specimens, 

 their capture just then and there can hardly be looked upon as accidental. 



As I have pointed out elsewhere (Bigelow, 1917, p. 248), "the appearance of 

 the Arctic Oikopleura in the gulf is especially noteworthy, since it has not been 

 recorded previously on this side of the Atlantic south of Baffins Bay, though known in 

 European waters as far south as the Shetland Islands (Lohmann, 1896 and 1901). 

 Thanks to Lohmann's excellent descriptions and figures (1896, p. 72, Taf. 14, figs. 6, 

 7, and 10; 1901, p. 15, figs. 16 and 17), it is easily recognized, its chief difference from 

 the closely allied 0. labradorien.sis being the presence of many small dendritic chordal 

 cells. Its very large size (rump length upward of 4 millimeters) is likewise diagnostic, 

 while the red margin of the tail makes it a conspicuous object in the water." 



" Mertensia occurred over the outer half of the continental shelf oft" Shelburne, Nova Scotia, on Mar. 19, 1320 (p. 371). 



