256 Mr. J. W. Fewkes on Deep-sea Medusce. 



evolutionist would expect from the uniformity of conditions 

 "which exist in deep water we find manifested in the simple 

 anatomy of two of the more characteristic deep-sea genera of 

 Acraspeda, a simplicity of structure of embryonic and there- 

 fore of ancestral nature. It is certainly strange that these 

 two facts are associated. It is an extraordinary coinci- 

 dence if the deep water at which the Medusa were found 

 and the embryonic affinities in their anatomy have not the 

 relationship of Cause and Effect. The discovery of a Nau- 

 jyhanta in the icy waters of the Arctic zone *, while it shows 

 that the genus may approach the surface when the temperature 

 of the depth at which it lives becomes a surface-temperature, 

 would also indicate that the genus is not confined to the great 

 depth at which it is reported from the South Atlantic. If 

 Nauphanta cannot rise to the surface in the latitudes of Tris- 

 tan d'Acunha, it may be that the elevation of temperature 

 above its habitat keeps it at great depths. At the higher 

 latitude of North Greenland, however, the cold zone, in which 

 Nauphanta lives in the South Atlantic, is about the surface- 

 temperature. Here then, as far as thermal conditions go, the 

 Medusa can rise to the surface. We here encounter what I 

 believe will be found to be an influence of more important 

 character in the modification of Medusan life at great depths 

 than the depth of water itself. Medusas are sensitive to 

 changes of temperature in the ocean ; so sensitive, in fact, that 

 for many genera the lines of demarcation between warm and 

 cold oceanic currents are often dead lines to these delicate 

 creatures. It is well known that certain genera can be frozen 

 without being killed by the change, and that Medusas suffer 

 less from a diminution in temperature than from an elevation 

 of the same. This is particularly true of those genera, like 

 Aurelia, ISarsia, and others, which habitually inhabit cold 

 water. A temperature of +70° F. is fatal to them, while 

 many tropical forms will easily live even in higher tempera- 

 tures. I'emperature in the ocean has drawn invisible lines 

 in the distribution of Medusas in depth as well as latitude ; 

 and it is at present very difficult to separate this cause from 

 that of pressure in the bathymetrical limits of the jelly-fishes. 

 The poverty of our knowledge of the ranges of temperatures 

 which jelly-fishes can endure is too great to admit of any 

 generalizations of value on this question. Still there are no 

 tacts of more vital importance in the discussion of the ques- 

 tion of whether there are deep-sea Acraspeda than those 



* ' Report on the jNIedusse collected by the Lady Franklin Bay Expe- 

 dition,' Lieut. A. W. Greely commanding. Appendix no. xi. 



