elf (fig. 29). The arterial system is very completely developed in
both Limulus and Scorpio, branching repeatedly until minute arterioles
are formed, not to be distinguished from true capillaries; these open
into irregular swollen vessels which are the veins or venous sinuses.
A very remarkable feature in Limulus, first described by Owen, is the
close accompaniment of the prosomatic nerve centres and nerves by
arteries, so close indeed that the great ganglion mass and its
out-running nerves are actually sunk in or invested by arteries. The
connexion is not so intimate in Scorpio, but is nevertheless a very
close one, closer than we find in any other Arthropods in which the
arterial system is well developed, e.g. the Myriapoda and some of the
arthrostracous Crustacea. It seems that there is a primitive tendency
in the Arthropoda for the arteries to accompany the nerve cords, and a
"supra-spinal" artery--that is to say, an artery in close relation to
the ventral nerve cords--has been described in several cases. On the
other hand, in many Arthropods, especially those which possess
tracheae, the arteries do not have a long course, but soon open into
wide blood sinuses. Scorpio certainly comes nearer to Limulus in the
high development of its arterial system, and the intimate relation of
the anterior aorta and its branches to the nerve centres and great
nerves, than does any other Arthropod.

[Illustration: FIG. 2l.--Development of the lateral eyes of a
scorpion. h, Epidermic cell-layer; _mes_, mesoblastic connective
tissue; n, nerves; II, III, IV, V, depressions of the epidermis in
each of which a cuticular lens will be formed.

(_From Korschelt and Heider, after Laurie_.)]

[Illustration: FIG. 22.--Section through the lateral eye of
_Euscorpius italicus_.

lens, Cuticular lens.
nerv c, Retinal cells (nerve-end cells).
rhabd, Rhabdomes.
nerv f, Nerve fibres of the optic nerve.
int, Intermediate cells (lying between the bases of the retinal
cells).

(After Lankester and Bourne from Parker and Habwell's _Text book of
Zoology_, Macmillan & Co.)]

[Illustration: FIG. 23.--Section through a portion of the lateral eye
of Limulus, showing three ommatidia--A, B and C. hyp, The epidermic
cell-layer (so-called hypodermis), the cells of which increase in
volume below each lens, l, and become nerve-end cells or
retinula-cells, rt; in A