324 BULLETIN OF THE BUREAU OF FISHERIES 



Canadian waters on the surface in 31 to 32 per mille when a few fathoms sinking 

 would have carried it into much more saline water. 



From the data just outlined it would appear that the whole column of water in 

 the offshore parts of the Gulf of Maine offers an environment favorable for the exist- 

 ence if not for the reproduction of S. serratodentata during the season (July to Sep- 

 tember) when it is most widespread there, but probably it could not long survive 

 water much less saline than about 31 per mille or colder than 6° to 8°, and Huntsman 

 (1919) has suggested that low salinity may be the factor that bars it from the Gulf 

 of St. Lawrence. 



Neither temperature nor salinity offers an explanation for the disappearance 

 of S. serratodentata from the gulf in autumn, for the water is considerably warmer 

 in November than when it first enters the gulf in spring, and the salinity is not very 

 different from that of late summer. Neither does its immigration into the gulf in 

 spring parallel the vernal warming of the water, but is not at its height until long after 

 the gulf is warm enough for its support. It is therefore likely that the increase in its 

 numbers with the summer chiefly mirrors an accumulation of the stock within the gulf, 

 where it finds good feeding ground and conditions favorable for growth and prolonged 

 existence. Apparently no more enter after early autumn, a phenomenon probably 

 connected with the seasonal reproductive cycle of the species, and as the visitors of 

 summer die off during the autumn from one cause or another or are devoured by 

 other animals without leaving progeny to take their places, S. serratodentata disap- 

 pears from the gulf, not to reappear there until with the earliest immigration of the 

 succeeding spring. 



Our data do not allow a statement as to the vertical distribution of S. serratodentata 

 in the Gulf of Maine more definite than that it has seldom been detected there at the 

 surface, though most often in hauls from shoaler than 100 meters. If it is actually 

 as uncommon right at the top of the water in the gulf as now appears to be the case, 

 the food supply may be as effective a factor as any of the physical features of its 

 surroundings in holding so rapacious an animal at lower levels. 



There is no evidence that this chsetognath ever succeeds in reproducing itself in 



the gulf. 



Sagitta maxima Conant 



In a previous chapter (p. 64) I have discussed the geographical distribution of 

 this species and of the next within the gulf from the standpoint of their routes of 

 entrance and dispersal. What demands chief emphasis here is that both S. maxima 

 and S. lyra are distinctly seasonal in the inner parts of the gulf, like S. serratodentata. 

 During all our cruises we have found only a single specimen of S. maxima within the 

 offshore banks during the summer or early autumn months (eastern basin, September 

 2, 1915, station 10310), our failure to find it there in July and August, 1914, being 

 specially significant because it occurred then off the seaward slope of Georges Bank 

 (station 10220). Neither have we any early winter records for it in the gulf; this, 

 however, may be an accident, for we have tried only two tows in the deep trough in 

 December or January, which may simply have missed the S. maxima. However, 

 this large chsetognath was detected at 12 stations within the gulf as well as over the 

 deeper parts of the continental shelf off southern Nova Scotia during March, April, 



