eres in
the head and the Arachnida only two, and Limulus agrees with the
Arachnida in this respect and differs from the Crustacea. The central
nervous systems of Limulus and of Scorpio present closer agreement in
structure than can be found when a Crustacean is compared with either.
The wide divarication of the lateral cords in the prosoma and their
connexion by transverse commissures, together with the "attraction" of
ganglia to the prosomatic ganglion group which properly belong to
hinder segments, are very nearly identical in the two animals. The
form and disposition of the ganglion cells are also peculiar and
closely similar in the two. (See Patten (42) for important
observations on the neuromeres, &c., of Limulus and Scorpio.)

[Illustration: FIG. 12.--The prosomatic appendages of _Limulus
polyphemus_ (right) and Scorpio (left), _Palamnaeus indus_ compared.
The corresponding appendages are marked with the same Roman numeral.
The Arabic numerals indicate the segments of the legs.

cox, Coxa or basal segment of the leg.
stc, The sterno-coxal process or jaw-like up-growth of the coxa.
epc, The articulated movable outgrowth of the coxa, called the
epi-coxite (present only in III of the scorpion and III, IV and V
of Limulus).
ex^1, The exopodite of the sixth limb of Limulus.
a, b, c, d, Movable processes on the same leg (see for some
suggestions on the morphology of this leg, Pocock in _Quart. Journ.
Micr. Sci._ March 1901; see also fig. 50 below and explanation).

(From Lankester, _loc. cit._)]

[Illustration: FIG. 13.--Diagrams of the metasternite st, with genital
operculum op, and the first lamelligerous pair of appendages ga, with
uniting sternal element st of Scorpio (left) and Limulus (right).

(From Lankester, _loc. cit._)]

2. _The Minute Structure of the Central Eyes and of the Lateral
Eyes._--Limulus agrees with Scorpio not only in having a pair of
central eyes and also lateral eyes, but in the microscopic structure
of those organs, which differs in the central and lateral eyes
respectively. The central eyes are "simple eyes," that is to say, have
a single lens, and are hence called "monomeniscous." The lateral eyes
are in Limulus "compound eyes," that is to say, consist of many lenses
placed close together; beneath each lens is a complex of protoplasmic
cells, in which the optic nerve terminates. Each