528 PYCNOGONIDA 



seem to show are not with the lower Arachnids but with the 

 higher ; they are either degenerates from very advanced and 

 specialised Arachnida, or they are lower than the lowest. Con- 

 fronted with such an issue, we cannot but conclude to let the 

 Pycnogons stand apart, an independent group of Arthropods x ; 

 and I am inclined to think that they conserve primitive features 

 in the usual presence of generative apertures on several pairs of 

 lirnbs, and probably also in the non-development of any special 

 respiratory organs. But inasmuch as the weight of evidence goes 

 to show that subservience of limbs to mouth is a primitive 

 Arthropodan character, the fact that the basal elements of the 

 anterior appendages have here (as in Koenenia) no such relation 

 to the mouth must be taken as evidence, not of antiquity, but 

 of specialisation. In like manner the suctorial proboscis cannot 

 be deemed a primitive character, and the much reduced abdomen 

 also is obviously secondary and not primitive. 



Classification. ISTo single genus more than another shows 

 signs of affinity with other groups, and no single organ gives us, 

 within the group, a clear picture of advancing stages of com- 

 plexity. On the contrary, the differences between one genus and 

 another depend very much on degrees of degeneration of the 

 anterior appendages, and we have no reason to suppose that these 

 stages of degeneration form a single continuous series, but have 

 rather reason to believe that degeneration has set in independently 

 in various ways and at various points in the series. But while 

 we are unable at present to form a natural classification 2 of the 

 Pycnogons, yet at the same time a purely arbitrary or artificial 

 classification, conveniently based on the presence or absence of 

 certain limbs, would run counter to such natural relationships 

 as we can already discern. 



1 Cf. Oudemans, Tijdschr. d. Ned. Dierk. Fer. (2), i., 1886, p. 41 : " Jedermann 

 weiss nun, dass diese Tiere cine ganz besondere Urgruppe bilden, ohne alle 

 Verwandschaft niit irgend einer anderen Arthropodengruppe." 



2 Cole (Ann. Mag. Nat. Hist. (7), xv., 1905, pp. 405-415) has attempted such 

 a phylogenetic classification, starting with Dccolopoda, and leading in two 

 divergent lines, through Nymphon and Pallene to the Pycnogonidae, and through 

 Eurycide and Ammothea to Colossendeis. This hint is in part adopted in the 

 subjoined classification. Bouvier, in his recent Report on the Pycnogons of the 

 French Antarctic Expedition (t. cit.), gives reasons for separating the Decolopodidae 

 and Colossendeidae from all the rest. Loman, in Die Pantopoden der Siboga- 

 Expedition, 1908, has recently suggested another, and in many respects novel, 

 classification of the whole group. 



