TEN-LEGGED PANTOPODS 99 



A striking example of generic convergence is 

 exhibited in the ten-legged Pantopods or "sea- 

 spiders " which have been obtained from the 

 South Polar region by the English and Scottish 

 National Antarctic Expeditions. They belong 

 to two quite distinct genera, Pentanymphon 

 Hodgson, 1904, and Decolopoda Eights, 1837; 

 the former only differing from Nymphon by its 

 possession of an extra pair of legs ; the latter 

 most nearly related to Colossendeis, from which 

 it differs likewise by its possession of an extra 

 pair of legs, and also by the presence of a 

 pair of well-developed, three-jointed mandibles. 

 These two Antarctic genera are thus nearly 

 related to two other representative Pantopod 

 genera but not to each other. Professor D'Arcy 

 Thompson 1 places them in widely separated 

 families : Decolopodidae and Nymphonidse ; and 

 Schimkewitsch 2 has arrived at the same con- 

 clusion, namely, that the ten-legged Pantopods 

 do not constitute a distinct group in contrast 

 with the more usual eight-legged forms, but 

 that they arose independently from eight-legged 

 forms, the number of legs giving no clue to 

 their affinities. 



The following table, compiled from the data 

 furnished by Hodgson, 3 who collected and 



1 D'Arcy W. Thompson, " Pycnogonida," Camb. Nat. Hist., iv., 

 1909, pp. 531 and 537. 



2 W. Schimkewitsch, " Uber die Periodicitat in dem System der 

 Pantopoden," Zool. Anz.^ xxx., 1906, p. 3. 



3 Besides the Reports of the Voyages, see the following papers 



