ccepted that the resemblances arise from close genetic relationship.
The view that Limulus, the king-crab, is an Arachnid was maintained as
long ago as 1829 by Strauss-Durckheim (1), on the ground of its
possession of an internal cartilaginous sternum--also possessed by the
Arachnida (see figs. 1, 2, 3, 4, 5 and 6)--and of the similarity of the
disposition of the six leg-like appendages around the mouth in the two
cases (see figs. 45 and 63). The evidence of the exact equivalence of
the segmentation and appendages of Limulus and Scorpio, and of a number
of remarkable points of agreement in structure, was furnished by Ray
Lankester in an article published in 1881 ("Limulus an Arachnid,"
_Quart. Journ. Micr. Sci._ vol. xxi. N.S.), and in a series of
subsequent memoirs, in which the structure of the entosternum, of the
coxal glands, of the eyes, of the veno-pericardiac muscles, of the
respiratory lamellae, and of other parts, was for the first time
described, and in which the new facts discovered were shown uniformly to
support the hypothesis that Limulus is an Arachnid. A list of these
memoirs is given at the close of this article (2, 3, 4, 5 and 13). The
Eurypterines (Gigantostraca) were included in the identification,
although at that time they were supposed to possess only five pairs of
anterior or prosomatic appendages. They have now been shown to possess
six pairs (fig. 47), as do Limulus and Scorpio.

[Illustration: FIG. 2.--Ventral surface of the entosternum of _Limulus
polyphemus_, Latr. Letters as in fig. 1 with the addition of NF, neural
fossa protecting the aggregated ganglia of the central nervous system;
PVP, left posterior ventral process; PMP, posterior median process.
Natural size.

(From Lankester.)]

[Illustration: FIG. 3.--Entosternum of scorpion (_Palamnaeus Indus_, de
Geer); dorsal surface.

asp, Paired anterior process of the sub-neural arch.
snp, Sub-neural arch.
ap, Anterior lateral process (same as RAP and LAP in fig. 1).
lmp, Lateral median process (same as ALR and PLR of fig. 1).
pp, Posterior process (same as PLP in fig. 1).
pf, Posterior flap or diaphragm of Newport.
m^1 and m^2, Perforations of the diaphragm for the passage of
muscles.
DR, The paired dorsal ridges.
GC, Gastric canal or foramen.
AC, Arterial canal or foramen.

(After Lankester, _loc. cit._)]

The various comparisons previously made between the structure of Limulus
and the Eurypterines on the