374 PEOF. E. E. LANKBSTEE ON CEETAIN POINTS IN THE 



occifamts (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 (PI. LXXXI. figs. 1 & 2 cc) 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 (PI. 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 Limuliis, 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 lacunae ; 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 lamellae of the lung-books, and finding its way 

 between the lamellae, as shown in PI. 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 (svsl, PI. LXXX. 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, PI. LXXX. fig. 1, where only five pairs are seen, and cv, in PI. LXXXI. 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- 

 Ciirdium, 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, 1 cannot say I have detected) would prevent the distending circumpulmonary 



