370 GEOGRAPHICAL DISTRIBUTION. 



is independent of the origin of the waters, Atlantic or Arctic, in which it is found. 

 But to say as they do that it is independent of temperature is true only if we 

 mean that its horizontal occurrence irrespective of vertical distribution is 

 independent of slight differences in surface temperature. As a matter of fact 

 there is good evidence that D. ardica is limited in dispersal chiefly by temperature. 

 It is a form adapted to cold waters; and in truly Arctic regions, such as the Green- 

 land Sea, is found indifferently on the surface, and in the intermediate depths. 

 Further to the south, where the surface temperature is higher, as for example 

 along the coast of Norway and in the Skagerak, it is exclusively confined to the 

 mesoplankton. At the present moment it is not possible to state precisely 

 what its temperature limits are. But it is known from only a few degrees above 

 the freezing point of salt water, and it has never been found regularly in water 

 warmer than 42° (Skagerak, 200 m., August). 



A few species of Siphonophores are known to occur throughout a wide 

 range of temperature. The best known member of this group, Diphyes appen- 

 diculata, is common on the surface throughout the Tropical Pacific (p. 248, 

 Agassiz and Mayer, : 02, p. 160) in temperatures of 78°, 80° or over. In the Atlan- 

 tic it is common in the high surface temperatures of the tropics, e. g. among the 

 West Indies, and is of very general occurrence. In this ocean it is known from 

 much lower temperature than any as yet recorded for it from the Pacific. In 

 the Mediterranean, Chun ('87) has recorded it from temperatures as low as about 

 56°, and in the Bay of Biscay it was abundant at 53°-52° (surface to 100 fathoms), 

 and was apparently not only alive but reproducing itself at a temperature of 

 about 50°. D. appendiculata has never, so far as I can learn, been taken in any 

 numbers in water colder than this, though once recorded from below 45° (Chun 

 '97b, p. 110, 60° 2' N., 22° 7' W.; closing-net, 800-1,000 meters). And since 

 we now have so many records of the constituents of the surface Plankton of 

 the boreal Atlantic, it is probably safe to say that its normal temperature range 

 does not extend much, if any, below 50°. 



Physophora hydrostatica, among Physophorae, occurs throughout a range 

 of temperatures almost as great as those occupied by D. appendiculata. But unlike 

 the latter, Physophora has been but seldom recorded from temperatures above 

 70°. The only recent records of this species from tropical regions which I have 

 been able to find are the Canaries, 1° 1' N., 16°40' W. ("Plankton" expedition), 

 and Malaysian region (Lens and Van Riemsdijk), But the Canary records 

 are all from the winter months (Chun, '88, Haeckel, '69a), when the surface 

 temperature is from 65° -68°; the "Plankton" record is from an intermediate 



