CH^TOGNATHA OF THE SCOTTISH NATIONAL ANTARCTIC EXPEDITION. 981 



these colder waters is responsible for this very sharp division, and it must also be 

 remembered that if collections had been made from the depths of the tropical and 

 sub-tropical Atlantic these would naturally have yielded cold-water forms. Yet, 

 despite these facts, this sharp line dividing the present collection into two distinct 

 groups serves as a very clear example of how great a part temperature plays in the 

 distribution of plankton organisms. 



In the Scotia collections made south of 40° S. there are found five species of 

 Chsetognatha — Sagitta gazellse, Sagitta maxima, Sagitta planctonis, Evkrohnia 

 hamata, and possibly a species of Heterokrohnia. These are all typically cold-water 

 forms ; their presence in these latitudes was to be expected, and adds nothing to our 

 knowledge of their distribution, all these species having been found by the German 

 South Polar Expedition (1901-03) under very much the same conditions. Eukrohnia 

 fowle^^i is a type which one might have expected to meet with, but it was not present 

 in this collection. Sagitta hexaptera, Sagitta serratodentata, and Krohnitta subtilis 

 have all been reported from the seas south of 40° S., but they do not seem to be typical 

 Antarctic species, only occasional specimens being got. None of these was present in 

 the Scotia material from this area. 



The remaining species, seven in number, were all taken between roughly 36° N. 

 lat. and 38° S. lat., and were thus living under tropical and sub-tropical conditions. 

 Many of the specimens were small and evidently immature, thus supporting 

 Gunther's view that large, mature individuals occur at some distance below the 

 surface, only small and mostly immature specimens being obtained from the very 

 uppermost layers of the sea. 



No species new to science was discovered, the only noteworthy feature of the 

 collection being the very large size of four specimens of Sagitta gazellse. These four 

 specimens seem to be the largest recorded for the species, and the largest of all 

 is probably the largest " Arrow Worm " that has been captured up to the present 

 time. Detailed measurements of this specimen are given under the notes of this 

 species. 



The difficulty of identifying Chsetognatha has within recent years been considerably 

 lessened by the excellent papers of Fowler and Eitter-Zahony on this class. These 

 have been of the greatest service, and I desire to express my great obligation to these 

 two authors. In selecting features for aid in identification Fowler has been followed 

 in his introduction to the Chsetognatha of the Biscayan Plankton (1905), where a most 

 helpful discussion of this difficult subject is given. As a rule, tables of measurements 

 and numbers of jaws and teeth have been given only when the species has not been 

 frequently described, or where there might be any doubt as to the identification. It 

 seemed to be fruitless to expand these notes unduly by including figures which had 

 been given frequently before, and which were quite typical and did not add to our 

 knowledge of the species in any way. It may be mentioned that the measurement of 

 length does not include the tail fin ; as this was so frequently damaged, it seemed to be 



