s
it always does the genital apertures. But it is necessary to remember,
in the light of recent discoveries, that the sixth prosomatic pair of
appendages is carried on the seventh somite of the whole series, there
being two prosthomeres or somites in front of the mouth, the first
carrying the eyes, the second the chelicerae; also that the first
mesosomatic or genital somite is not the seventh or even the eighth of
the whole senes of somites which have been historically present, but
is the ninth, owing to the presence or to the excalation of a
praegenital somite. It seems that confusion and trouble will be best
avoided by abstaining from the introduction of the non-evident
somites, the ocular and the praegenital, into the numerical
nomenclature of the component somites of the three great body regions.
We shall, therefore, ignoring the ocular somite, speak of the first,
second, third, fourth, fifth and sixth leg-bearing somites of the
prosoma, and indicate the appendages by the Roman numerals, I, II,
III, IV, V, VI, and whilst ignoring the praegenital somite we shall
speak of the first, second, third, &c., somite of the mesosoma or
opisthosoma (united mesosoma and metasoma) and indicate them by the
Arabic numerals.

[Illustration: FIG. 10.--Ventral view of a scorpion, _Palamnaeus
indus_, de Geer, to show the arrangement of the coxae of the limbs,
the sternal elements, genital plate and pectens.

M, Mouth behind the oval median camerostome.
I, The chelicerae.
II, The chelae.
III to VI, the four pairs of walking legs.
VIIgo, The genital somite or first somite of the mesosoma with the
genital operculum (a fused pair of limbs).
VIIIp, The pectiniferous somite.
IXstg to XIIstg, the four pulmonary somites.
met, The pentagonal metasternite of the prosoma behind all the
coxae.
x, The sternum of the pectiniferous somite.
y, The broad first somite of the metasoma.]

There are a number of other important points of structure besides
those referring to the somites and appendages in which Limulus agrees
with Scorpio or other Arachnida and differs from other Arthropoda. The
chief of these are as follows:--

[Illustration: FIG. 11.--Third leg of _Limulus polyphemus_, showing
the division of the fourth segment of the leg by a groove S into two,
thus giving seven segments to the leg as in scorpion.

(From a drawing by P