6 G. HERBERT FOWLER. 



III. DISTRIBUTION. 



The occurrence of these species in really polar waters is of considerable interest. 



The record of Krohnia hamata as a truly " bipolar " species is thus completed ; 

 it ranges from 81 30' N. (Homer and Schaudiun) to 77 49' S. ; it is both epiplanktonic 

 and mesoplanktonic at high latitudes with low surface temperatures, but in tropical and 

 warm temperate seas is known only from the colder strata of the mesoplankton. 

 While, horizontally speaking, it appears to be cosmopolitan, and is fairly eury thermal, 

 it has not been recorded definitively from a higher temperature than 12 '7 Cent. 

 (' Challenger ' Collection). 



As to Smjitta hexaptera, the captures of both ' Discovery ' and ' Challenger ' 

 complete its record as a " pantothermal " species, that is to say, one living at practically 

 every known sea-temperature, from about 29 C. to 2 C. ; and as a truly cosmopolitan 

 species, ranging from Spitsbergen in the north to the ' Discovery's ' winter quarters 

 in the south, and from every sea from which Chaetognatha have been recorded. 



Sayitta serratodentata, on the other hand, failed at the colder stations of both 

 ' Discovery ' and ' Challenger' ; its minimum record f has now been reduced to 4 '6 C., 

 the lowest temperature at which it had been taken previously being between 6 ' 6 and 

 12*2C. at the Falkland Islands. Its definitive temperature limit in the Northern 

 Hemisphere is at present 12'2 C., but in view of the 'Discovery's' captures it may 

 possibly prove to extend somewhat further north than its present record of lat. 

 60 12' N. At any rate, it seems to be clear that it is not " bipolar," although 

 subantarctic and north -temperate. 



The southern captures of hexaptera, serratodentata, and hamata, from all sources,- 

 have been plotted on the appended circumpolar chart, together with the mean annual 

 surface isotherms of 12 and 6 C., as calculated by Dr. Schott. The former appears 

 to indicate approximately the northerly limit of hamata as an epiplanktonic form, the 

 latter similarly to indicate the southerly limit of serratodentata. 



The definitive temperature limits given in the ' Siboga ' Report already cited 

 require amendment as follows : hexaptera, lowest temperature, 2 C. ; serratodentata, 

 lowest temperature, 4'6C. ; hamata, highest surface temperature, 12'7C., lowest 

 temperature, 2 C. 



t This record is, however, of a single specimen, and the species had failed at the two previous stations 

 (compare p. 3 above). It is therefore possible that we are dealing merely with an isolated specimen, which had 

 drifted bevond its natural limits. 



