200 CRUSTACEA 



111 the Antarctic zone we are presented with very different 

 relations, since the great continents are drawn out to points 

 towards tlie south, and are isolated by vast tracts of intervening- 

 deep sea. ISTevertheless, certain littoral forms are circumpolar, 

 e.g. the Palinurid lasus and tlie Crabs Cyclograpsus and Hymeno- 

 soma. The genus Dromidia is common to Australia and South 

 Africa, though it is apparently absent from South America. 



The Isopod genus Serolis is confined to Antarctic seas. The 

 majority are littoral species, and they are distributed round the 

 coasts of • Patagonia, Australia, and Kerguelen in a manner tliat 

 certainly suggests a closer connection between these shores in the 

 past. These facts are, on the whole, evidence in favour of the 

 former existence of an Antarctic continent stretching farther 

 north and connecting Australia, Africa, and S. America — a 

 supposition that has been put forward to account for the dis- 

 tribution of the Penguins, Struthious birds, Oligochaets, Crayfishes, 

 etc., in these regions (.see pp. 215-217). 



In considering the Arctic and Antarctic faunas the supposed 

 phenomenon of bipolarity must be mentioned, i.e. the occurrence 

 of particular species in Arctic and Antarctic seas, but not in the 

 intermediate regions. This discontinuous type of distribution 

 was upheld for a variety of marine animals by Pfeffer, Murray, 

 and others, but it has been very adversely criticised by 

 Ortmann.^ As far as the Arctic and Antarctic Decapod fauna 

 in general are concerned, the north polar forms are quite distinct 

 from the south polar. Typical of the former are Hirp-polytc, 

 Sclerocrangon, Hyas, Homarus, etc. ; of the latter, Hyinenosotna, 

 Dromidia, lasus. It appears, however, that in certain special 

 cases, bipolarity of distribution may be produced owing to the 

 operation of peculiar causes. Two such cases seem to be fairly 

 well established. C'rangon antarcticus occurs at the two poles, 

 and apparently not in the intermediate regions ; but, as Ort- 

 mann points out, it is represented right down the West American 

 coast by a very closely related form, C. franciscormn. The 

 waters on the tropical western coasts both of Africa and America 

 are exceedingly cool, and it appears that in this way the Crangon 

 may have migrated across the tropical belt, leaving a slightly 

 modified race to represent it in this intermediate region. The 

 other case of bipolarity is afforded by the " Schizopod," Boreo- 



' American Naturalist, xxxiii., 1899, ]i. .583. 



