GEOGRAPHICAL DISTRIBUTION. 375 



temperature. Chun ('97b) has already suggested this explanation, and pointed 

 out that the currents in the Straits of Gibraltar would offer no barrier to the 

 entrance of surface organisms, but just the reverse. The surface water at the 

 Canaries where the typical tropical species are common in winter, never falls 

 below about 65.5°, whereas in the western half of the Mediterranean, the surface 

 temperature falls in winter to about 56°. This is far colder than any waters 

 in which the tropical Siphonophores are regular characteristics of the surface 

 plankton. And it is significant that Diphyopsis dispar and Agahna okeni 

 were absent from waters below 68° in the Eastern Pacific. We can say in gen- 

 eral that none of the tropical species are constant members of the plankton in 

 any regions where the surface waters fall below about 65° at any season, though 

 they may occur sporadically, or even more or less regularly far to the north 

 within the sweep of the Gulf Stream, or any other warm current. 



Several tropical species, e. g. D. dispar, Bassia bassensis, and Ceratocymba, 

 have been observed in the Straits of Gibraltar, as we might expect from the fact 

 that the surface flow is into the Mediterranean. And it is probable, though not 

 certain, that the Physalias which occasionally appear in the Bay of Naples are 

 casual visitors from the Atlantic. The failure of these forms to establish theni- 

 selves in the Mediterranean was correctly explained by Chun ('97b, p. 109) as 

 due to the differences in temperature, which "der Reife der Geschlechtsprodukte 

 von atlantischen und in das Mittelmeer eingeschwimmten Arten hinderlich in 

 Wege stehen" is probably a correct explanation. Whether adult, or as larvae, 

 they can not survive the winter of the western Mediterranean. 



At present we have no evidence that any of the mesoplanktonic Sipho- 

 nophores, i. e. Chuniphyes or the Bathyphysinae, have penetrated into the 

 Mediterranean. And if further research proves that such is the case, the absence 

 could be explained by the currents in the Straits of Gibraltar. It has long been 

 known that the lower strata flow out of the Mediterranean into the Atlantic, 

 and the recent work of the "Michael Sars" (Hjort, :11) has shown that there 

 is no inflow below about 75 fathoms at any stage of the tide; and that the 

 maximal outflow at 100-175 fathoms may reach a velocity of two meters per 

 second. These conditions would be quite sufficient to exclude deep-water 

 Siphonophores from the Mediterranean had they occupied the intermediate 

 waters since that sea acquired its modern oceanographic features and were they 

 absolutely limited to the intermediate depths throughout their existence. But 

 it is quite possible that they have been overlooked in the Mediterranean, for 

 they are, apparently, luicommon everywhere. 



