II is here certainly the tergum of the first somite of
the mesosoma--the genital somite--and is _not_ a survival of the
embryonic praegenital somite. The anus (not seen) is on the sternal
surface.

(From Lankester, _loc. cit._)]

It is important to note that the series of lamellae of the lung-book
and the gill-book correspond _exactly_ in structure, the narrow, flat
blood-space in the lamellae being interrupted by pillar-like junctions
of the two surfaces in both cases (see Lankester (4)), and the free
surfaces of the adjacent lamellae being covered with a very delicate
chitinous cuticle which is drawn out into delicate hairs and
processes. The elongated axis which opens at the stigma in Scorpio and
which can be cleared of soft, surrounding tissues and coagulated blood
so as to present the appearance of a limb axis carrying the book-like
leaves of the lung is not really, as it would seem to be at first
sight, the limb axis. That is necessarily a blood-holding structure
and is obliterated and fused with soft tissues of the sternal region
so that the lamellae cannot be detached and presented as standing out
from it. The apparent axis or basal support of the scorpion's
lung-books shown in the figures, is a false or secondary axis and
merely a part of the infolded surface which forms the air-chamber. The
maceration of the soft parts of a scorpion preserved in weak spirit
and the cleaning of the chitinized in-grown cuticle give rise to the
false appearance of a limb axis carrying the lamellae. The margins of
the lamellae of the scorpion's lung-book, which are _lowermost_ in the
figures (fig. 15) and appear to be free, are really those which are
attached to the blood-holding axis. The true free ends are those
nearest the stigma.

Passing on now from the mesosoma we come in Scorpio to the metasoma of
six segments, the first of which is broad whilst the rest are
cylindrical. The last is perforated by the anus and carries the
post-anal spine or sting. The somites of the metasoma carry no
parapodia. In Limulus the metasoma is practically suppressed. In the
allied extinct Eurypterines it is well developed, and resembles that
of Scorpio. In the embryo Limulus (fig. 42) the six somites of the
mesosoma are not fused to form a carapace at an early stage, and they
are followed by three separately marked metasomatic somites; the other
three somites of the metas