ocock.)]

1. _The Composition of the Head_ (that is to say, of the anterior part
of the prosoma) _with especial Reference to the Region in Front of the
Mouth._--It appears (see ARTHROPODA) that there is embryological
evidence of the existence of two somites in Arachnida which were
originally post-oral, but have become prae-oral by adaptational
shifting of the oral aperture. These forwardly-slipped somites are
called "prosthomeres." The first of these has, in Arachnids as in
other Arthropods, its pair of appendages represented by the eyes. The
second has for its pair of appendages the small pair of limbs which in
all living Arachnids is either chelate or retrovert (as in spiders),
and is known as the chelicerae. It is possible, as maintained by some
writers (Patten and others), that the lobes of the cerebral nervous
mass in Arachnids indicate a larger number of prosthomeres as having
fused in this region, but there is no _embryological_ evidence at
present which justifies us in assuming the existence in Arachnids of
more than two prosthomeres. The position of the chelicerae of Limulus
and of the ganglionic nerve-masses from which they receive their
nerve-supply, is closely similar to that of the same structures in
Scorpio. The cerebral mass is in Limulus more easily separated by
dissection as a median lobe distinct from the laterally-placed ganglia
of the chelceral somite than is the case in Scorpio, but the relations
are practically the same in the two forms. Formerly it was supposed
that in Limulus both the chelicerae and the next following pair of
appendages were prosthomerous, as in Crustacea, but the dissections of
Alphonse Milne-Edwards (6) demonstrated the true limitations of the
cerebrum, whilst embryological researches have done as much for
Scorpio. Limulus thus agrees with Scorpio and differs from the
Crustacea, in which there are three prosthomeres--one ocular and two
carrying palpiform appendages. It is true that in the lower Crustacea
(Apus, &c.) we have evidence of the gradual movement forward of the
nerve-ganglia belonging to these palpiform appendages. But although in
such lower Crustacea the nerve-ganglia of the third prosthomere have
not fused with the anterior nerve-mass, there is no question as to the
prae-oral position of two appendage-bearing somites in addition to the
ocular prosthomere. The Crustacea have, in fact, three prosthom