68 

 Sagitta bipunctata Quoy et Gaimard. 



According to the recorded occurrences of this species, it would seem to be equally at 

 home in a brackish estuary or in the centre of a main ocean, at a mean annual temperature ot 

 about o° in the White Sea to 33 in the Red Sea, and from 82°W. to 1 20 E. If this be correct, 

 bipunctata would be about the most eurythermal, euryhaline, and cosmopolitan organism known. 



Against such a distribution there is no reasonable argument to allege, but, frankly, I am 

 quite unable to believe it without more evidence. It can only be urged in defence of such 

 incredulity that I know bitterly how difficult it is to distinguish between bipunctata and the young 

 of several other species; this has no such unmistakeable features as some species present. 



The wide extension of the records is due mainly to Levinsen (86) and Steinhaus (96). 

 Of these, Levinsen furnishes only a record of geographical positions without further data. 

 Steinhaus, who shares with Strodtmann the credit of the only recent attempts to gauge the 

 distribution of Chaetognatha, dealt with several collections, of which some were well, others 

 badly, preserved; among the latter were numerous Indo-pacific records of bipunctata, which I 

 suspect on the ground that this species was not captured by Mr Gardiner in many months 

 work in the Maldive Archipelago, nor by the Siboga in a year of regular townettings; this is 

 sufficiently remarkable in an ocean which is stated to have so uniform a plankton-fauna as the 

 Indo-pacific. As has already been pointed out (pp. 16, 17) there is every excuse for confusing 

 bipunctata and badly preserved neglecta, which occurs as far north as Japan, and as far south 

 as Java; and the explanation may possibly lie in this fact. It is greatly to be hoped that with 

 the energy displayed at the present time in collecting abroad, and with the increased use of 

 formalin, the extreme records may be either confirmed or refuted. In the meantime it appears 

 to be premature to attempt to define the horizontal distribution of this species, and a waste 

 of labour to plot the captures on a Chart. 



It is, however, generally agreed that bipunctata is a neritic or coast-wise form, and that 

 its oceanic occurrences are rather exceptional (Strodtmann, 92 p. 15; Steinhaus, 96 pp. 28,39; 

 Fowler, 05 p. 69). It appears to be euryhaline, and to tolerate a low salinity in the Zuijder See, 

 East Scheldt, at nearly the longitude of Memel in the Baltic, and in the mouth of the Para River. 



As regards the vertical distribution, Steinhaus records no less than 7 specimens from 

 a closing net 850 — 650 m., at fairly high temperatures (9°4 to [i2°5J); but it failed at greater 

 depths and lower temperatures. In the Research it failed entirely in all the 35 hauls with the 

 closing net which began at 365 m. or a greater depth; that is to say below the isothermobath 

 of 1 i° C, which tallies well with Steinhaus' record. 



The mere fact that (so far as our evidence at present goes) this temperature appears 

 approximately to set its depth-limit, forms a certain amount of evidence against its alleged 

 occurrence in the far north, and suggests that the unnamed recorder for the Russian section 

 of the Conseil International 04 (1) may have been dealing really with arctica. 



of the epiplankton of an archipelago; for the strong tides and currents generally met with in such areas must produce nearly the same 

 epiplanktonic conditions throughout. With the mesoplankton, however, below the action of tide and current, the case is somewhat different: 

 one would not class hamata, for example, as neritic, merely because it occurred in the deep water of an archipelago. The epiplankton 

 of the Siboga area has therelore been regarded in this place as neritic, the apparently purely mesoplanktonic species as oceanic. 



