J. W. Fewkes — Deep-Sea Medusas. 175 



dence if the deep water at which the medusae were found 

 and the embryonic affinities in their anatomy have not the re- 

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

 jphanta in the icy waters of the Arctic,* 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 

 JVauphanta 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 lati- 

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

 JVauphanta 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 be- 

 lieve will be found to be an influence of more important char- 

 acter in the modification of medusan life at great depths, than 

 the depth of water itself. Medusae are sensitive to changes of 

 temperature in the ocean ; so sensitive in fact, that for many 

 genera the line of demarkation 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 medusae suffer less from a dim- 

 inution in temperature than from an elevation of the same. 

 This is particularly true of those genera like Aurelia, Sarsia 

 and others which habitually inhabit cold water. A tempera- 

 ture of —{—70° F., is fatal to them, while many tropical forms 

 will easily live even in higher temperatures. Temperature in 

 the ocean has drawn invisible lines in the distribution of me- 

 dusae in depth as well as latitude, and it is at present very dif- 

 ficult to separate this cause from that of pressure in the bathy- 

 metrical limits of the jelly-fishes. The poverty of our knowl- 

 edge of the ranges of temperatures which jelly-fishes can en- 

 dure is too great to admit of any generalizations of value on 

 this question. Still there are no facts of more vital importance 

 in the discussion of the question of whether there are deep- 

 sea Acraspeda than those which bring information of the ther- 

 mal limits at which the medusae can live. 



It would be profitable, if space permitted, to consider other 

 genera of Acraspeda made known by the Albatross, in their 

 bearings on the question which is the title of this paper. 

 The three genera already considered present us the strongest 

 arguments which can be found in the modification of external 

 and internal anatomy, as indicative of a deep-sea habitat. 



* Report on the Medusae collected by the Lady Franklin Bay Expedition, Lieut. 

 A-. W. G-reely commanding. Appendix No. XL 



