60 
BULLETIN OF THE BUREAU OF FISHERIES 
It was for only a brief period, however, that these Arctic animals persisted in the 
plankton of the gulf during the spring in question, for none of them were captured 
there during our later cruises (June to October) that year, except for the single Mer- 
tensia just mentioned; and although Mertensia, Limacina, and Oikopleura van- 
hoffeni were all present over or outside the continental shelf abreast of Cape Sable as 
late as June 24, available data suggest that the planktonic species of this category 
disappear, from west to east, successively, from the coast water between Cape Sable 
and Halifax with the advance of the summer, as I have noted elsewhere (Bigelow, 
1917, p.249). 
Whether the Gulf of Maine is annually invaded by these species is yet to be deter- 
mined, but what little is known of the seasonal expansion and contraction of the 
Fig. 32. — Localities at which certain planktonic animals of Arctic origin were taken in May and June, 1915. H, Limacina 
helicina ; M, Mertensia ovum; O, Oikopleura mnhojjeni; P, Ptychogena ladca 
Nova Scotian current makes this seem probable. Nor does the fact that the more 
delicate of the Arctic planktonic animals are scarce, if not absent, from the gulf in 
any given summer mean that no such invasion occurred during the year in question, 
for Mertensia (A. Agassiz, 1865) is extremely sensitive to water that is too warm. 
And since, judging from my own experience, this applies equally to Limacina helicina 
and to the Arctic Oikopleura, it is only while a direct and considerable influx of 
northern water is taking place around Cape Sable into the gulf (distinguished from 
the increment it contributes to the general inflowing drift) that they are likely to 
appear in the catches of the tow nets. Consequently, failure to find them in mid- 
summer has no bearing on their presence or absence a month or two earlier in the 
season. 
