anal. In each somite of the mesosoma is a
small, free entosternite having a similar position, but below or
ventral to the nerve cords, and having a smaller number of muscles
attached to it. The entosternite was probably in origin part of the
fibrous connective tissue lying close to the integument of the sternal
surface--giving attachment to muscles corresponding more or less to
those at present attached to it. It became isolated and detached, why
or with what advantage to the organism it is difficult to say, and at
that period of Arachnidan development the great ventral nerve cords
occupied a more lateral position than they do at present. We know that
such a lateral position of the nerve cords preceded the median
position in both Arthropoda and Chaetopoda. Subsequently to the
floating off of the entosternite the approximation of the nerve cords
took place in the prosoma, and thus they were able to take up a
position below the entosternite. In the mesosoma the approximation had
occurred before the entosternites were formed.

[Illustration: FIG. 17.--Embryo of scorpion, ventral view showing
somites and appendages.

sgc, Frontal groove.
sa, Rudiment of lateral eyes.
obl, Camerostome (upper lip).
so, Sense-organ of Patten.
PrGabp^1, Rudiment of the appendage of the praegenital somite which
disappears.
abp^2, Rudiment of the right half of the genital operculum.
abp^3, Rudiment of the right pecten.
abp^4 to abp^7. Rudiments of the four appendages which carry the
pulmonary lamellae.
I to VI, Rudiments of the six limbs of the prosoma.
VIIPrG, The evanescent praegenital somite.
VIII, The first mesosomatic somite or genital somite.
IX, The second mesosomatic somite or pectiniferous somite.
X to XIII, The four pulmoniferous somites.
XIV, The first metasomatic somite.

(After Brauer, _Zeitsch. wiss. Zool_., vol. lix., 1895.)]

In the scorpion (figs. 3 and 4) the entosternite has tough
membrane-like outgrowths which connect it with the body-wall, both
dorsally and ventrally forming an oblique diaphragm, cutting off the
cavity of the prosoma from that of the mesosoma. It was described by
Newport as "the diaphragm." Only the central and horizontal parts of
this structure correspond precisely to the entosternite of Limulus:
the right and left anterior processes (marked ap in figs. 3 and 4, and
RAP, LAP, in fi