386 GENERAL CONSIDERATIONS. 



GENERAL CONSIDERATIONS. 



As our knowledge of the geographic distribution of the various groups of 

 pelagic Metazoa grows, it becomes increasingly profitable to compare them with 

 one another. One field of discussion which has led to many interesting results 

 in other groups, the relationship between Arctic and Antarctic faunae, is closed 

 to us, because no Antarctic Siphonophore is yet known. But we have enough 

 data to allow some important comparisons. 



The qualitative richness of the Siphonophore fauna of warm waters as 

 compared to its poverty in cold latitudes is paralleled by Medusae (Maas, : 06) 

 Ctenophores (Moser, :09), Pteropods (Meisenheimer, :08), Chaetognaths (Fowler, 

 :06), Salpae (Apstein, :08) and by various other groups which share with Sipho- 

 nophores a permanently pelagic existence, and helplessness to alter their environ- 

 ment by directive horizontal swimming. 



The Siphonophores are as a whole restricted to an extremely uniform envi- 

 ronment; much more so than the Medusae. Thus no Siphonophore has pene- 

 trated into brackish, much less into fresh water. Even in oceanic regions of 

 low salinity Siphonophores are not common. Not one has adopted the bottom 

 as its usual habitat, as have certain Medusae ; none are parasitic, while Medusae 

 are parasitic on molluscs, and on each other (various Narcomedusae). And like 

 the cold-water members of several of these groups, the boreal and Arctic Siphono- 

 phores are not structurally primitive. They certainly are not ancestral types. 

 For genera or families which might be looked on in such a light, e. g. Monophyids 

 and Prayids among Calycophores, Apolemia among Physophores, we must 

 turn to warmer zones. And on the other hand none of the highly specialized 

 Siphonophores are known from cold waters. Thus there is no Arctic Hippopo- 

 did, no Anthophysid, no Rhizophysalid, no Chondrophorid. .\i-guing from such 

 grounds Maas (:06), Meisenheimer (:05, :08) and Moser (:09) have reached 

 the very important generalization that for Medusae, Pteropods, and Cteno- 

 phores, the centre of development lies in warm or in temperate seas, and that 

 they have spread thence into cold zones to the north or south, where they may 

 or may not have become differentiated specifically. The Medusae have likewise 

 made an incursion, important both numerically and physiologically, into the 

 peculiar though uniform envkonment provided by the intermediate water 

 layers. And there is every reason to extend these generalizations to the Siphono- 



