874 PROF. E. R. LANKESTER ON CERTAIN POINTS IN THE 
occitanus (the Spanish Yellow Scorpion). It shows excellently the lateral arteries and 
the whole series of pericardio-ventral muscles. ‘The blood is pumped by the contractile 
heart lying within the pericardium (Pl. LXXXI. figs. 1 & 2cc) into these lateral 
arteries, also into anterior and posterior arteries. The anterior arteries especially 
accompany the great nerves, and one main trunk is completely reflected ventrally and 
accompanies the nerve-cords throughout the length of the animal, giving off lateral 
branches (Pl. LXXXI. figs. 1 & 2 spa). 
The arteries thus arising branch very abundantly and supply directly every organ, 
even every muscle, in the body. The finest branches of these arteries are entitled to 
be termed capillaries. I have described them and the similar vessels in Limulus, in 
my article on the skeleto-trophic tissues of these animals, in the Quart. Journ. Micr. 
Sci. for January 1884. 
The capillaries of the Scorpion (and the same is true for other large Arthropoda, 
such as the Crayfish) do not reunite, as in Vertebrates, to form a tree of branches 
which gradually increase in bulk, but they open into more or less irregular spaces, 
often large and shallow, which surround the chief organs. These may be called, as is 
the custom, sinuses or lacunz; but they are truly veins with their own proper walls, 
though of non-cylindrical form in cross-section. In the region of the prosoma and 
mesosoma these spaces open into the two large longitudinal ventral veins which have 
the lung-sacs sunk into them at intervals in the 3rd, 4th, 5th, and 6th mesosomatic 
segments. A large part of the blood arriving in these great ventral sinuses or veins 
will come in contact with the delicate lamelle of the lung-books, and finding its way 
between the lamellx, as shown in Pl. LXXXI. figs. 3 & 4, it will be subject to gas- 
exchange. From these longitudinal ventral trunks the blood then passes in a partially 
oxygenated condition by the superficial lateral veins (sus/, Pl. LX XX. figs. 1, 2, 3) into 
the pericardium, from whence it is taken by the expanding heart (expanding by the 
elasticity of its walls after contraction) through its seven pairs of valvular apertures 
(vv, Pl. LXXX. fig. 1, where only five pairs are seen, and cv, in Pl. LX XXII. fig. 2) into 
its cavity and again sent on its round. ‘The main force at work in drawing the blood 
from the cicumpulmonary sinuses of the longitudinal ventral veins into the peri- 
cardium, is clearly enough (as in other Arthropods) the same contraction of the 
heart which expels the blood through the arteries. ‘The contraction of the heart 
creates a diminution of the tension in the pericardium. But there can be no doubt 
that both in Limulus and in Scorpio the pericardio-ventral (veno-pericardiac) muscles 
exercise an important influence in drawing the blood from the general venous space- 
system surrounding the viscera into the circumpulmonary sinuses. These muscles 
probably contract simultaneously with the contraction of the heart, and thus, while 
tending to keep the pericardium distended, also distend the circumpulmonary sinuses, 
and cause a rush of blood into those chambers. Valvular arrangements (which, how- 
ever, I cannot say I have detected) would prevent the distending circumpulmonary 
