PLANKTON OF THE GULF OF MAINE 63 



Stream, it is not direct overflows or influxes of the latter across the offshore banks 

 that maintain the large stock within the gulf during its season of abundance, but 

 the general indraft of mixed water. 



The euphausiid shrimp Nematoscelis megalops (p. 146), which is less common 

 than S. serratodentata in the inner parts of the gulf but is equally characteristic of 

 the upper strata of water along the continental slope, occupies the same faunal 

 status. 



The large and easily recognized clurtognath Eulrohnia Jiamata (p. 328) is a 

 characteristic inhabitant of a lower level in the mixed water (say, below 50 meters), 

 though not of the deepest. Its faunal relationship is diametrically opposite to that of 

 its relative, S. serratodentata, for while it is widely dispersed over the ocean basins 

 in the mid-depths, it is only in the Arctic or at least in cold seas that it comes to the 

 surface regularly (Apstein, 1911). It enters the Gulf of Maine by the same route 

 followed by S. serratodentata, but below it, and is equally unable to breed within the 

 gulf, 30 though in its case this failure is because the temperatures it experiences there 

 are too high instead of too low. 



The eastern channel entrance to the gulf is deep enough to include a part of the 

 vertical zone in which this species is most plentiful in the mixed water over the slope, 

 where it appears in considerable numbers between 100 and 300 meters as well as 

 deeper (p. 329, and Huntsman, 1919) ; hence it is not surprising that it should occur 

 commonly in our deeper hauls in the gulf though seldom on the surface. The vary- 

 ing sizes of the individuals taken there suggest that it is able to "carry on" through- 

 out its natural span of life anywhere in the gulf below, say, 100 meters, though 

 unable to reproduce. 



Our records do not show the migration routes for Eukrohnia as clearly as they 

 do for Sagitta serratodentata, because the former is a year-round member of the 

 plankton of the gulf. For this reason (coupled, as I believe, with longevity within 

 the gulf), it is to be expected anywhere within our limits below 100 or 150 meters and 

 at any season, though the extreme southwest corner of the deep basin off Cape Cod 

 and also certain isolated sinks to which its access is more or less obstructed, may prove 

 exceptions to this rule. If all our records of Eukrohnia for all seasons are united, 

 however, there is a decided preponderance in the eastern, and particularly the ex- 

 treme northeastern, parts of the gulf contrasted with its western side, not only in the 

 number of stations at which it has been taken but also in its local abundance, which 

 agrees with the general anticlockwise direction of the inflowing eddy. The distribu- 

 tion of Eukrohnia (p. 328) illustrates how closely its inward route follows the Eastern 

 Channel and the slope of Browns Bank. Although Eukrohnia is a constant con- 

 stituent of the plankton all along the seaward slope of Georges Bank, the latter must 

 by its shoalness, oppose an absolute barrier to its dispersal, for we have not found a 

 single specimen at any of our stations on the bank at any season. Consequently, 

 none of the Eukrohnia that have passed the mouth of the Eastern Channel as they 

 drift westward can enter the gulf on their farther journey. Finally, I may point 

 out that the regularity with which Eukrohnia appears in the gulf is as good evidence 



30 Although Gulf of Maine specimens are oiten large, we have found none there with sexual organs developed. 

 75898—26 5 



