. In Scorpio the
completion of the horizontal plate by oblique naps, so as to form an
actual diaphragm shutting off the cavity of the prosoma from the rest
of the body, possibly gives to the organs contained in the anterior
chamber a physiological advantage in respect of the supply of arterial
blood and its separation from the venous blood of the mesosoma.
Possibly the movement of the diaphragm may determine the passage of
air into or out of the lung-sacs. Muscular fibres connected with the
suctorial pharynx are in Limulus inserted into the entosternite, and
the activity of the two organs may be correlated.

5. _The Blood and the Blood-vascular System._--The blood fluids of
Limulus and Scorpio are very similar. Not only are the blood
corpuscles of Limulus more like in form and granulation to those of
Scorpio than to those of any Crustacean, but the fluid is in both
animals strongly impregnated with the blue-coloured respiratory
proteid, haemocyanin. This body occurs also in the blood of Crustacea
and of Molluscs, but its abundance in both Limulus and Scorpio is very
marked, and gives to the freshly-shed blood a strong indigo-blue tint.

[Illustration: FIG. 20.--View of the ventral surface of the mid-line
of the prosomatic region of _Limulus polyphemus_. The coxae of the
five pairs of limbs following the chelicerae were arranged in a series
on each side between the mouth, M, and the metasternites, mets.

sf, The sub-frontal median sclerite.
Ch, The chelicerae.
cam, The camerostome or upper lip.
M, The mouth.
pmst, The promesosternal sclerite of chitinous plate, unpaired.
mets, The right and left metasternites (corresponding to the
similarly placed pentagonal sternite of Scorpio). Natural size.

(After Lankester.)]

The great dorsal contractile vessel or "heart" of Limulus is closely
similar to that of Scorpio; its ostia or incurrent orifices are placed
in the same somites as those of Scorpio, but there is one additional
posterior pair. The origin of the paired arteries from the heart
differs in Limulus from the arrangement obtaining in Scorpio, in that
a pair of lateral commissural arteries exist in Limulus (as described
by Alphonse Milne-Edwards (6)) leading to a suppression of the more
primitive direct connexion of the four pairs of posterior lateral
arteries and of the great median posterior arteries with the heart
its