8 FJiWARD T. BROWXE. 



In the coUectiou from the FalkhmcLs tlieie are seventeen species of Hydromedusae (for 

 names, see Browne, 1902) belonging to sixteen genera. Not one of these species has 

 yet been found in the Antarctic, and only two of the genera, namely, Eleutheria and 

 Phialidium, are represented there. Among the Scyphomeduste the genus Desmonema 

 is common to the Magellanic and Antarctic regions, but the species are distinct. 

 Eleutheria charcuti, found oft" the Antiirctic continent near Wandel Lsland (south of 

 the Falklands), is more like E. hodgsoni from McMurdo Sound than like E. vallentini 

 from the Falklands. If we compare the Antarctic Medusae with the records (which 

 are still very meagre) from Australia and New Zealand, we find that only one genus 

 [Marijelopsis) and no species are common to both regions. 



The recent Autiirctic explorations have produced a fair number of new Medusaj, 

 many of which have well-marked and interesting specific characters, but there are only 

 about three new genera. I expect that ultimately not one of them will remain 

 peculiar to the Antarctic fauna. All the genera, except those recently described, have 

 representatives in other parts of the world, frequently living under totally diflferent 

 conditions and in localities far apart. As the littoral Hydromedusai of the Antarctic 

 have not yet been found in the Magellanic, South Australian, and New Zealand areas, 

 it looks as if they l)elonged to an ancient stock which has long been isolated by the 

 Great Southern Ocean from the rest of the world. 



Sir John Murray, K.C.B. (1896), says: "In water of a low temperature the 

 metabolism in cold-blooded animals would Ije much less rapid than in water of a 

 high temperature, and all those cliangcs which result in the evolution of new species 

 would proceed at a much slower rate at the poles than in the tropical belt." If the 

 Medusae of the Antarctic region have long been isolated, and their evolution has 

 proceeded at a slow rate on account of the coldness of the water, then, when an Antarctic 

 species is compared with another species of the same genus inhabiting warmer water 

 we ought to be able to see a diff"erence and mark the course of evolution. As evolution 

 is proceeding at a much slower rate in cold than in warm regions, the characters of 

 an Antarctic Medusa should be more primitive than those of one from warmer seas. 



The following are instances of this primitive condition : — 



The genus Solinundella has a very wide geographical range, extending from the 

 tropics to the Antarctic. It has only two opposite pcrradial tentacles, and the genus 

 is descended, without doubt, from a genus which had four perradial tentacles. Beneath 

 the two tentacles there is always a deep groove in the wall of the umbrella. In the 

 Antarctic form there is still a conspicuous groove present in the two perradii without 

 tentacles. The grooves have disappeared from the two perradii without tentacles in 

 the species found oft" Ceylon. The species from Ceylon has not only lost all traces of 

 the grooves, but in addition has developed about four times the number of sense 

 organs found in the Antarctic species. 



A new .species of Silmgita found in the Antarctic lias only four centripetal amals, 

 whereas the other species have eight or more centripetal canals. 



