cardium passes. There are seven pairs of these
_veno-pericardiac vertical muscles_ in Scorpio, and eight in Limulus
(see figs. 30, 31, 32). It is obvious that the contraction of these
muscles must cause a depression of the floor of the pericardium and a
rising of the roof of the ventral blood sinus, and a consequent
increase of volume and flow of blood to each. Whether the pericardium
and the ventral sinus are made to expand simultaneously or all the
movement is made by one only of the surfaces concerned, must depend on
conditions of tension. In any case it is clear that we have in these
muscles an apparatus for causing the blood to flow differentially in
increased volume into either the pericardium, through the veins
leading from the respiratory organs, or from the body generally into
the great sinuses which bring the blood to the respiratory organs.
These muscles act so as to pump the blood through the respiratory
organs.

[Illustration: FIG 25.--Section through one of the central eyes of a
young Limulus.

L, Cuticular or corneous lens.
hy, Epidermic cell-layer.
corn, Its corneagen portion immediately underlying the lens.
ret, Retinula cells.
nf, Nerve fibres.
con. tiss, Connective tissue (mesoblastic skeletal tissue).

(After Lankester and Bourne, _Q. J. Mic. Sci._, 1883.)]

It is not surprising that with so highly developed an arterial system
Limulus and Scorpio should have a highly developed mechanism for
determining the flow of blood to the respiratory organs. That this is,
so to speak, a need of animals with localized respiratory organs is
seen by the existence of provisions serving a similar purpose in other
animals, e.g. the branchial hearts of the Cephalopoda.

The veno-pericardiac muscles of Scorpio were seen and figured by
Newport but not described by him. Those of Limulus were described and
figured by Alphonse Milne-Edwards, but he called them merely
"transparent ligaments," and did not discover their muscular
structure. They are figured and their importance for the first time
recognized in the memoir on the muscular and skeletal systems of
Limulus and Scorpio by Lankester, Beck and Bourne (4).

6. _Alimentary Canal and Gastric Glands._--The alimentary canal in
Scorpio, as in Limulus, is provided with a powerful suctorial pharynx,
in the working of which extrinsic muscles take a part. The mouth is
relatively