77 



It will be apparent from the table that the species of Chaetognatha are, so far as we 

 know, by no means of the universal distribution which some writers have assigned to them. 

 As already mentioned, even such a widely ranging form as serratodentata apparently fails to 

 penetrate into the colder parts of the Arctic Ocean. But further conclusions as to epiplanktonic 

 distribution are most safely left, till the results of the National, Valdivia, and the various 

 Antarctic expeditions have appeared. 



As regards the Mesoplankton, no less than ten species (omitting bipunctatd) have been 

 shown definitively by closing nets to penetrate beyond the horizon of ioo fathoms (182 m.) ; to 

 these may with fair probability be added Sibogae, although known only from deep Vertical Nets. 



Of these eleven, four — namely decipiens, macrocephala, Whartoni, and Zetesios, — 

 have so far been found in the mesoplankton alone. Two more — arctica and hamata, the 

 former arctic, the latter bipolar, — appear to seek deep water at latitudes with high surface 

 temperatures; and there is some evidence (p. 70) for thinking that furcata also maybe reckoned 

 in this category. Of subtilis, serratodentata and hexaptera, there is reason to believe that, 

 although markedly eurythermal, they do not descend to such extreme depths as do some other 

 Chaetognaths. 



The last table (p. 78) summarises in comparative form the temperature range of the valid 

 species (omitting bipunctatd), based on definitive observations. Where an exact temperature is not 

 available, the symbol > implies more than, the symbol < less than, the temperature opposite 

 to which the wider part of the symbol is set : thus, for example, arctica shows a possible 

 minimum of o°5, and a possible maximum of io°. 



It would seem that we have here at least four classes : 



(a) cold-water species with a maximum of (say) 1 3 : 

 arctica, macrocephala, Whartoni, Zetesios, hamata ; 



(b) temperate species : 

 furcata, cephaloptera ; 



(c) warm-water species, with a minimum of (say) 1 6° : 



Bedoti, enflata, ferox, minima, neglecta, planctonis, pulchra, regularis, robusta, pacifica, draco; 



(d) eurythermal species : 

 hexaptera, serratodentata, subtilis. 



Of decipiens, elegans and Sibogae it is not possible to make any statement in this 

 connection. 



Of course these limits will probably be extended in some degree as our knowledge 

 increases; it is possible that some warm-water or temperate species may enter the eurythermal 

 class, or even that one of the latter class may leave it; but, little as we know at present, it is 

 fairly safe to prophesy that the cold-water species will not be blended with the warm-water forms. 



So far as they go, the definitive observations show no evidence against the idea that 

 temperature is the chief determinant in the distribution; but, unfortunately, it would be still 

 premature to attempt to map the oceans into zoological areas for epiplanktonic Chaetognaths, 



