388 GENERAL CONSIDERATIONS. 



Even in temperate, "mischgebiete" regions Siphonophores are compara- 

 tively unimportant members of the plankton, from the quantitative point of 

 view, although, as I have already pointed out (p. 369), the number of species is 

 already much greater than it is in colder waters. Thus in the Bay of Biscay for 

 example, Mr. Fowler found only one Siphonophore regularly enough for me to 

 plot its vertical distribution. But when we turn to the metazoan plankton of 

 tropical and subtropical waters, we find Siphonophores relatively very much 

 more important. It is true that they seldom fill the surface waters to the extent 

 that Salpae often do ; but there are few pelagic forms more regular in their occur- 

 rence in warm currents than Diphyes appendiculata, Diphyopsis dispar, or 

 Abylopsis tetragona. And the swarms of Velella and Porpita have long attracted 

 notice. 



The fact that the warm-water Siphonophores are divided into two main 

 groups, one restricted to characteristically tropical temperatures, i. e. above 

 about 65°; the other with a range wide enough to allow them to occupy the 

 Mediterranean, is likewise significant, because no such division can be made 

 for the holoplanktonic Medusae, the Pteropods (Meisenheimer) , nor Salpae 

 (Apstein). 



When we come to compare the intrusion of Siphonophores into the inter- 

 mediate water layers with that of the Medusae, we find a state of affairs very 

 similar to the differences in their extensions into Arctic regions. Medusae have 

 been very successful colonists of deep water; they are surpassed by fishes alone 

 in the diversity of the ancestral stocks which have sent offshoots into this en- 

 vironment. Thus among the Craspedotae no less than seven families, including 

 Antho-, Lepto-, Tracho-, and Narcomedusae, have representatives among the 

 mesoplankton ; while the Scyphomedusae are represented by six families. On 

 the other hand there are only two families of Siphonophores which have members 

 belonging exclusively to the mesoplankton, and no one of these occurs as regu- 

 larly or as commonly as do several of the " intermediate " Medusae, for example 

 Colobonema or Halicreas. 



In the poverty of their mesoplanktonic constituents the Siphonophores 

 agree with Ctenophores (Moser, : 09). These various facts taken together point 

 in the first place to the conclusion that Siphonophores are as a whole much more 

 sensitive to temperature than either Medusae, Ctenophores, or Pteropods, and, 

 in the second, that they have been less able to adapt themselves to changes in 

 temperature, and to occupy the oceanic zones which would thus have been opened 

 to them. They are, too, far more sensitive to differences in salinity (p. 380). 



