PYCNOGONIDA— OALMAN. 7 



results when the Pycuogonida are compared with one another. Of all Pycnogonida 

 hardly any can be less " vagile," as adults or as larvae, than the species of Pycnogonum ; 

 yet not only P. gaini mentioned above, but also the common P. llttorale of our own 

 coasts, show that the species may combine a very wide geographical range with a great 

 constancy of specific characters. 



VI.-THE SIGNIFICANCE OF THE DECAPODOUS 



PYCNOGONS. 



Although the present collection contains no species that throws new light on the 

 major problems connected with the morphology and phylogeny of the Pycnogonida, 

 it may not be out of place here to make a few observations on points raised in 

 Prof Bouvier's Report on the Pycnogonida of the " Pourquoi Pas ? " 



Prof. Bouvier pays the compliment of serious criticism to a little essay (1909), 

 in which I supported the view (first put forward by Prof G. H. Carpenter) that the 

 decapodous condition among Pycnogons is not a primitive survival but a recent 

 specialisation. The argument on which I chiefly relied was based on the fact that 

 Decolopoda and Pentanymphon., the only decapodous genera then known, are by no 

 means nearly related to one another, but exhibit the closest affinity respectively with 

 Colossendeis and Nymphon, two of the normal octopodous genera. This argument 

 was greatly strengthened, as I have elsewhere pointed out (1910), by Prof Bouvier's 

 discovery of Pentapycnon, a decapodous genus widely removed from the other two, 

 but approximating very closely indeed to Pycnogonum ; and, while Decolopoda and 

 Pentanymphon can, without much difticulty, be admitted as reasonably primitive forms, 

 Pycnogonum and, with it, Pentapycnon, can only be regarded as among the most 

 highly specialised of existing Pycnogons. On the other hand, the support which my 

 contention seemed to draw from the fact that all three decapodous genera occurred only 

 within a restricted geographical area has been quite destroyed by Prof Bouvier's later 

 discovery of a species of Pentapycnon on the coast of French Guiana — one of the last 

 places in the world where one would look for a fauna with antarctic afiinities. 



Prof. Bouvier's argument for the primitive nature of the decapodous forms 

 depends, in the first place, on the admitted fact thut DecolojJoda is, in one respect 

 (apart from the number of somites), less specialised than its relative Colossendeis ; 

 it retains, in the adult state, the chelophores with a biarticulate scape that are present 

 only in the young stages in the last-named genus. Now it may be conceded that, if 

 Decolopoda stood alone, it might be "simpler and perfectly logical" to suppose that 

 Colossendeis had been derived from it by the loss of two primitive characters, the 

 chelophores and the posterior pair of legs ; but when we have to extend a similar 

 supposition to Pentanymphon and, still more, to Pentapycnon, the argument, though 



