developed besides the gill-book and its base; it is fused with its
fellow of the opposite side. On the other hand, in Scorpio, the
gill-book-bearing appendage has sunk below the surface, forming a
recess or chamber for itself, which communicates with the exterior by
an oval or circular "stigma" (fig. 10, stg). That this in-sinking
has taken place, and that the lung-books or in-sunken gill-books of
Scorpio really represent appendages (that is to say, limbs or
parapodia) is proved by their developmental history (see figs. 17 and
18). They appear at first as outstanding processes on the surface of
the body.

The exact mode in which the in-sinking of superficial outstanding
limbs, carrying gill-lamellae, has historically taken place has been a
matter of much speculation. It was to be hoped that the specimen of
the Silurian scorpion (_Palaeophonus_) from Scotland, showing the
ventral surface of the mesosoma (fig. 49), would throw light on this
matter; but the specimen recently carefully studied by the writer and
Pocock reveals neither gill-bearing limbs nor stigmata. The
probability appears to be against an actual introversion of the
appendage and its lamellae, as was at one time suggested by Lankester.
It is probable that such an in-sinking as is shown in the accompanying
diagram has taken place (fig. 15); but we are yet in need of evidence
as to the exact equivalence of margins, axis, &c., obtaining between
the lung-book of Scorpio and the gill-book of Limulus. Zoologists are
familiar with many instances (fishes, crustaceans) in which the
protective walls of a water-breathing organ or gill-apparatus become
converted into an air-breathing organ or lung, but there is no other
case known of the conversion of gill processes themselves into
air-breathing plates.

[Illustration: FIG. 7.--Diagram of the dorsal surface of _Limulus
polyphemus_.

oc, Lateral compound eyes.
oc', Central monomeniscous eyes.
PA, Post-anal spine.
I to VI, The six appendage-bearing somites of the prosoma.
VII, Usually considered to be the tergum of the genital somite, but
suggested by Pocock to be that of the otherwise suppressed
praegenital somite.
VIII to XIII, The six somites of the mesosoma, each with a movable
pleural spine and a pair of dorsal entopophysis or muscle-attaching
ingrowths.
XIV to XVIII, The confluent or unexpressed six