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 OiTcopleura van- 

 hoffmi 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 

 helicim, M, MeTtensia ovum; 0> Oikopleura vanhojteni; P, Ptychogeila lactea 



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 Jielicina. 

 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. 



