484. GEORGE H. CARPENTER. 
tracheal respiration, however, as in many other characters, 
the Solifugida are highly specialised. I must confess myself 
unable to understand how some students of Arachnid mor- 
phology have concluded that tracheal tubes preceded lung- 
books in the evolution of the class. 
The position assigned by Lankester to the Pycnogonida— 
within the Arachnid class, but at a lower grade than all the 
other orders from the Merostomata onwards—agrees closely 
with my own views as shown in the genealogical table which 
accompanies my recent paper (8, pl. vi). Pycnogons cer- 
tainly differ from typical Arachnids so markedly that their 
separation in a distinct sub-class, as proposed by Lankester, 
is abundantly justified. But their division into three orders, 
as proposed several years ago by Sars (27), and now, under 
new ordinal names, by Pocock (20, pp. 224-5) is open to 
serious objection. Despite differences in points of detail, the 
genera of Pycnogonida show such fundamental unity of 
structure combined with such varying relationships among 
themselves that it is best to comprise them all within a single 
order. Certainly their separation into ordinal groups formed 
on the presence or absence of the chelicere or the palps must 
lead to many unnatural associations. For example, the 
relationship of the Pallenide to the Nymphonide which is 
expressed by the scheme of Sars and Pocock is undeniable. 
But within the Pallenide must be included forms—Phoxi- 
chilidium and allies—with chelicerze variously developed and 
with palps vestigial or absent, which lead on towards 
Phoxichilus, a genus placed, however, by Sars and Pocock in 
a distinct “ order” 
total absence of chelicerze and palps, Phoxichilus and Pycno- 
gonum have little in common, and the loss of a particular 
pair of appendages might readily be sustained independently 
by two or more divergent genera of a degenerate group. 
along with Pycnogonum. Beyond the 
Ancestry of the Crustacea. 
If, then, the common ancestors of Arachnids, Crustaceans, 
Insects, Centipedes and Millipedes, possessed a definite and 
