THE PROBLEM OF THE PYCNOGONS 691 



one of the most central and unspecialised genera of the group ; 

 but Decolopoda departs widely from this type, and approximates 

 to the very different Colossendeis. Two explanations of this 

 state of things are possible. The first and most obvious is that 

 the ten-legged condition is a primitive character which has 

 been retained by the most primitive members in each of two 

 diverging branches from the ancestral stock of the Pycnogonida. 

 Something like this view is held by Bouvier, who assigns to 

 the one branch Decolopoda and Colossendeis (Colossendeomorpha). 

 and to the other Pentanymphon and all the other Pycnogons 

 (Pycnogonomorpha). Bouvier, however, admits that Penta- 

 nymphon is in some respects {e.g. in the number of segments 

 in the chelicerse and palps) less primitive than some of the 

 other genera assigned to the Pycnogonomorpha, and, as a 

 whole, the scheme of classification which he gives seems forced 

 and unnatural. 



Carpenter, faced by the difficulty which the ten-legged 

 species present to his scheme of homology between the 

 Arachnid and Pycnogonid appendages, suggests with some 

 hesitation that " the fifth pair of legs in these genera may 

 possibly represent a comparatively new development," and not 

 a primitive character — implying also, it is to be presumed, that 

 this development has arisen independently in the two cases. It 

 must be admitted that the constancy in the number of somites 

 and appendages throughout the comparatively wide range of 

 structure presented by the eight-legged Pycnogons strongly 

 suggests that this is the deep-seated and, so to speak, " normal" 

 plan of structure of the group ; while the fact that the deca- 

 podous condition appears in two genera, which have little else 

 in common, is in favour of the view that this is a secondary 

 modification of the original plan. 



There is one case among Crustacea, to which attention has 

 not hitherto been drawn in this connection, which appears to 

 bear a certain analogy to that of the ten-legged Pycnogons. 

 The genus Polyartemia, belonging to the order Anostraca in 

 the sub-class Branchiopoda, differs from all the other members 

 of the order in having nineteen somites between the head and 

 the " genital segment," which bears the openings of the repro- 

 ductive organs. In all the other Anostraca there are eleven 

 somites in this region of the body, and the very close resemblance 

 in other respects between Polyartemia and the rest of the 



