528 PYCNOGONIDA ’ CHAP. 
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 !; 
and I am inclined to think that they conserve primitive features 
in the usual presence of generative apertures on several pairs of 
limbs, 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 Aoenenia) 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——No 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 ® of the 
Pyenogons, 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. Ver. (2), i., 1886, p. 41: ‘*‘ Jedermann 
weiss nun, dass diese Tiere cine ganz besondere Urgruppe bilden, ohne alle 
Verwandschaft mit irgend einer anderen Arthropodengruppe.” 
2 Cole (Ann. Mag. Nat. Hist. (7), xv., 1905, pp. 405-415) has attempted such 
a phylogenetic classification, starting with Decolopoda, and leading in two 
divergent lines, through Nymphon and Pallene to the Pycnogonidae, and through 
Eurycide and Aimmothea 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 (¢. 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. 
