692 SCIENCE PROGRESS 



Anostraca suggests that the nineteen somites in the one case 

 correspond, as a whole, to the eleven somites in the other. 

 Further, in the other orders of the Branchiopoda (except the 

 Cladocera, which have the body very much shortened) the 

 genital openings are either on the eleventh or on the twelfth 

 trunk-somite, so that there is some ground for believing this to 

 be approximately the primitive position, from which Polyartemia 

 alone has departed. In other words, it seems as though the 

 number of pre-genital somites, which in the evolution of the 

 group had become established as a constant character, had 

 become unstable, and reached a new " position of equilibrium " 

 in the case of Polyartemia. If we may assume that something 

 analogous has happened in the case of Decolopoda and Penta- 

 nymphon — that they are, like Polyartemia, departures from the 

 normal line of evolution of the group to which they belong — 

 then it is permissible to leave them out of account in considering 

 the affinities of the group as a whole. 



Huxley wrote long ago that " in the absence of any adequate 

 palaeontological history of the Invertebrata, any attempt to 

 construct their Phylogeny must be mere speculation." So far 

 as the great bulk of the Arthropoda, at any rate, are concerned, 

 this adequate palaeontological history is still to seek. Never- 

 theless, some attempt to construct a Phylogeny — speculative 

 though it may remain — must be the basis of any attempt at a 

 natural scheme of classification ; and, in spite of the modern 

 tendency to confine biological research to matters which admit 

 of experimental or statistical verification, some approximation 

 to a natural classification is an intellectual necessity for the 

 biologist. 



Literature 



Bouvier, E. L., Pycnogonides du Frangais. Exped. Antarct. Frangaise : Sciences 



Naturelles, Documents scientifiques, pp. 69, 3 pis. 48 text figs. 1907. 

 Carpenter, G. H., Notes on the Segmentation and Phylogeny of the Arthropoda, 



with an account of the Maxillae in Polyxenus lagurus. Quart. Journ. Micro. 



Sci., xlix. pp. 469-91, pi. xxviii. 1905. 

 COLE, L. J., Ten-legged Pycnogonids, with Remarks on the Classification of the 



Pycnogonida. Ann. Mag. Nat. Hist. (7), xv. pp. 405-15, 1905. 

 Eights, J., Description of a New Animal belonging to the Arachnides of Latreille, 



discovered in the Sea along the shores of the New South Shetland Islands. 



Boston Journ. Nat. Hist., i. pp. 203-6, pi. vii. 1834-7. 

 Hodgson, T. V., On a New Pycnogonid from the South Polar Regions. Ann. 



Mag. Nat. Hist. (7), xiv. pp. 458-62, pi. xiv. 1904. 



