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 off 
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 gulf in our midsummer, autumn, winter, or early 
spring hauls except for an odd Mertensia 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 coelenterates, with 
the large shelled pteropod Limacina helicina and the appendicularian Oilcopleura 
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 . labradoriensis 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.” 
Jt Mertensia occurred over the outer half of the continental shelf off Shelburne, Nova Scotia, on Mar. 19, 1920 (p. 371) . 
