, the letters rh point to a rhabdomere
secreted by the cell rt; c, the peculiar central spherical cell; n,
nerve fibres; mes, mesoblastic skeletal tissue; ch, chitinous
cuticle.

(From Korschelt and Heider after Watase.)]

An arrangement of great functional importance in regard to the venous
system must now be described, which was shown in 1883 by Lankester to
be common to Limulus and Scorpio. This arrangement has not hitherto
been detected in any other class than the Arachnida, and if it should
ultimately prove to be peculiar to that group, would have considerable
weight as a proof of the close genetic affinity of Limulus and
Scorpio.

[Illustration: FIG. 24.--Diagrams of the development and adult
structure of one of the paired central eyes of a scorpion.

A, Early condition before the lens is deposited, showing the folding
of the epidermic cell-layer into three.
B, Diagram showing the nature of this infolding.
C, Section through the fully formed eye.
h, Epidermic cell-layer.
r, The retinal portion of the same which, owing to the infolding,
lies between gl, the corneagen or lens-forming portion, and pr, the
post-retinal or capsular portion or fold.
l, Cuticular lens.
g, Line separating lens from the lens-forming or corneagen cells of
the epidermis.
n, Nerve fibres.
rh, Rhabdomeres.

[How the inversion of the nerve-end-cells and their connexion with
the nerve-fibres is to be reconciled with the condition found in the
adult, or with that of the monostichous eye, has not hitherto been
explained.]

(From Korschelt and Heider.)]

The great pericardial sinus is strongly developed in both animals. Its
walls are fibrous and complete, and it holds a considerable volume of
blood when the heart itself is contracted. Opening in pairs in each
somite, right and left into the pericardial sinus are large veins,
which bring the blood respectively from the gill-books and the
lung-books to that chamber, whence it passes by the ostia into the
heart. The blood is brought to the respiratory organs in both cases by
a great venous collecting sinus having a ventral median position. In
both animals _the wall of the pericardial sinus is connected by
vertical muscular bands to the wall of the ventral venous sinus_ (its
lateral expansions around the lung-books in Scorpio) in each somite
through which the peri