HANSEN AND SÖRENSEN: THE ORDER PALPIGRADI. 2:37 
blypygi (with the family Phrynoide) and Uropygi (with its tribes 
Oxopoet — the family 7helyphonoide — and Tartarides) ex- 
hibit great mutual differences, for instance in the structure of the 
mouth, And indeed, it is the most central of the orders of 
Arachnida: through Amblypygi it reveals decided affinity with 
Aranee, through Tartarides with Palpigradi, and through Oxo- 
poet — to a minor degree — with Scorpiones and Chelonethr. 
(As stated by THORELL several years ago, Opzliones and Acari 
are closely related to each other, but we confess that these two 
as well as So/zfug@e must still be said to stand far apart from 
the others.) 
As far as we can see, the most essential external characters of 
the different orders of Arachnida must be taken from the structure 
of 1) the mouth, 2) the antennæ** (first pair of limbs), 3) the 
other limbs and 4) the (real) number of the abdominal segments. 
1. The mouth of the Palpigradi (as we have already point- 
ed out above) differs from that of all other Arachnida, nay 
from that of almost all other Condylopods in being formed exclusively 
by the labrum and the hypostoma. And even if Koenenia did 
not offer other characteristics, this circumstance would, to our 
eyes, be sufficient to set it aside as an independent order. 
2. The antennæ of the Pa/pigradi are 3-jointed, and the 
two distal joints form a pair of pinchers with horizontal move- 
ment of the third joint against the prolongation of the second. 
In this structural feature Aoenenia agrees with the Opiliones and 
Scorpiones and differs most decisively from the Peazpalpi in 
which the antennz are 2-jointed and do not form real pinchers. 
3. The five remaining pairs of limbs are similar in all the 
most essential features, as none of the anterior three pairs (2nd, 
3rd and 4th) are provided with maxillary lobes which help to 
form the mouth, but are developed exclusively as organs of mo- 
vement, like the two posterior pairs of limbs which in all Arach- 
nids —- where they are found at all are merely instruments 
of movement. Therefore the foot (tarsus s. lat.) in all five pairs 
is divided into a metatarsus and a tarsus (s. str.). — In the four 
*8 Only in the very extensive and most varying division of the Acari, 
the antennz show essential differences within the same order. 
15 
