482 GEORGE H. CARPENTER. 
the older authors did not consider the nature (i.e. whether 
primitively pre-oral or post-oral) of the cephalic somites, and 
that the head of an Arachnid is less complex than that of a 
Crustacean or an Insect. My own belief is that the older 
authors’ concept of a primitive Arthropod head with five 
limb-bearing somites (six if the eyes be regarded as appendi- 
cular), all of which are now known to have been originally 
post-oral, will yet be recognised as sound. Such a head 
clearly characterised the trilobites, and among them one 
somite only—that bearing the simple feelers of Triarthrus— 
had, in addition to the ocular somite, become pre-oral. These 
feelers must be compared with the antennules, not with the 
eye-stalks, of a typical Crustacean, and the foremost pair of 
a trilobite’s biramous head-limbs (on whose segment the 
mouth apparently opened) with the Crustacean antenne. 
This latter pair of ihmbs—behind which the mouth has shifted 
in Arachnids as in all recent Crustacea—are represented by 
the Arachnid cheliceree. Hmbryological evidence for this 
comparison—suggested by many students of Arthropod mor- 
phology—might be found in the vestigial feelers described 
by von Jaworowski (16) in the embryo of the spider Trochosa, 
but the appendicular nature of the structures seen by him is 
open to doubt. More probably the trilobitan antennules are 
represented by the paired rudiments whence the Arachnid 
rostrum arises. The nerves of the rostrum take their origin, 
according to Korschelt and Heider (17, vol. 3, p. 13), from a 
small unpaired section of the brain in front of the cheliceral 
ganglia and behind the large protocerebral lobes whence the 
optic nerves arise. Brauer’s description and figures of the 
developing scorpion’s brain (2) further confirm the view that 
the cheliceral ganglia are tritocerebral. 
The presence of three free leg-bearing somites in the Soli- 
fugida, the Palpigradi, and the Pycnogonida suggests that 
the arachnid cephalothorax has been formed by the union of 
three trunk-segments with the primitive Arthropodan head. 
If this suggestion be accepted it is found that the Arachnida 
agree exactly with the typical Insecta and Crustacea in the 
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