gs. 1 and 2) correspond in the two animals, and the
median lateral process _lmp_ of the scorpion represents the tendinous
outgrowths ALR, PLR of Limulus. The scorpion's entosternite gives rise
to outgrowths, besides the great posterior flaps, pf, which form the
diaphragm, unrepresented in Limulus. These are a ventral arch forming
a neural canal through which the great nerve cords pass (figs. 3 and
4, _snp_), and further a dorsal gastric canal and arterial canal which
transmit the alimentary tract and the dorsal artery respectively
(figs. 3 and 4, GC, DR).

[Illustration: FIG. 18.--Portion of a similar embryo at a later stage
of growth. The praegenital somite, VII PrG, is still present, but has
lost its rudimentary appendages; go, the genital operculum, left half;
Km, the left pecten; abp^4 to abp^7, the rudimentary appendages of
the lung-sacs.

(After Brauer, _loc. cit_.)]

In Limulus small entosternites are found in each somite of the
appendage-bearing mesosoma, and we find in Scorpio, in the only somite
of the mesosoma which has a well-developed pair of appendages, that of
the pectens, a small entosternite with ten pairs of muscles inserted
into it. The supra-pectinal entosternite lies ventral to the nerve
cords.

In Mygale (figs. 5 and 6) the form of the entosternite is more like
that of Limulus than is that of Scorpio. The anterior notch Ph.N. is
similar to that in Limulus, whilst the imbricate triangular pieces of
the posterior median region resemble the similarly-placed structures
of Limulus in a striking manner.

[Illustration: FIG. 19.--Section through an early embryo of _Limulus
longispina_, showing seven transverse divisions in the region of the
unsegmented anterior carapace. The seventh, VII, is anterior to the
genital operculum, op, and is the cavity of the praegenital somite
which is more or less completely suppressed in subsequent development,
possibly indicated by the area marked VII in fig. 7 and by the great
entopophyses of the prosomatic carapace.

(After Kishinouye, _Journ. Sci. Coll. Japan_, vol. v., 1892.)]

It must be confessed that we are singularly ignorant as to the
functional significance of these remarkable organs--the entosternites.
Their movement in an upward or downward direction in Limulus and
Mygale must exert a pumping action on the blood contained in the
dorsal arteries and the ventral veins respectively