one hand, and that of a typical Arachnid,
such as Scorpio, on the other, had been vitiated by erroneous notions as
to the origin of the nerves supplying the anterior appendages of Limulus
(which were finally removed by Alphonse Milne-Edwards in his beautiful
memoir (6) on the structure of that animal), and secondly by the
erroneous identification of the double sternal plates of Limulus, called
"chilaria," by Owen, with a pair of appendages (7). Once the identity of
the chilaria with the pentagonal sternal plate of the scorpion is
recognized--an identification first insisted on by Lankester--the whole
series of segments and appendages in the two animals, Limulus and
Scorpio, are seen to correspond most closely, segment for segment, with
one another (see figs. 7 and 8). The structure of the prosomatic
appendages or legs is also seen to present many significant points of
agreement (see figures), but a curious discrepancy existed in the
six-jointed structure of the limb in Limulus, which differed from the
seven-jointed limb of Scorpio by the defect of one joint. R.I. Pocock of
the British Museum has observed that in Limulus a marking exists on the
fourth joint, which apparently indicates a previous division of this
segment into two, and thus establishes the agreement of Limulus and
Scorpio in this small feature of the number of segments in the legs (see
fig. 11).

[Illustration: FIG. 4.--Ventral surface of the same entosternum as that
drawn in fig. 3. Letters as in fig. 3 with the addition of NC, neural
canal or foramen.

(After Lankester, _loc. cit._)]

It is not desirable to occupy the limited space of this article by a
full description of the limbs and segments of Limulus and Scorpio. The
reader is referred to the complete series of figures here given, with
their explanatory legends (figs 12, 13, 14, 15). Certain matters,
however, require comment and explanation to render the comparison
intelligible. The tergites, or chitinized dorsal halves of the body
rings, are fused to form a "prosomatic carapace," or carapace of the
prosoma, in both Limulus and Scorpio (see figs. 7 and 8). This region
corresponds in both cases to six somites, as indicated by the presence
of six pairs of limbs. On the surface of the carapace there are in
both animals a pair of central eyes with simple lens and a pair of
lateral eye-tracts, which in Limulus consist of closely-aggregated
simple eyes, forming a "compound" ey