804 CLASS V. ARACHNIDA. 



leads forwards and soon divides into two, from each of which 

 six vessels are given off to supply the prosomatic appendages, 

 and other vessels to the brain and organs of the prosoma. 

 Behind the point where it splits the truncus arteriosus gives off 

 a right and a left branch which encircle the alimentary canal 

 and fuse in the middle line ventrally ; these vessels are then 

 continued backward as a supra-neural artery which is in close 

 contact with the ventral nerve-cord (Fig. 520, 7). 



The blood system of Scorpio is better developed than is usually 

 the case in Arthropods. The arteries end in minute channels 

 not to be distinguished from capillaries, and these debouch into 

 veins or into spacious venous sinuses. Two of the largest of 

 these sinuses lie in the median line, one dorsal and one ventral. 

 The former is the space of the pericardium ; its walls are fibrous 

 and not fenestrated and the blood is returned to it by special 

 veins, a pair in each somite, which bring the oxygenated blood 

 back from the lung- books. From the pericardium it enters the 

 ostia of the heart. The blood is collected for transmission to 

 the gills by the ventral venous sinus, and the roof of this sinus 

 is connected with the floor of the pericardium by seven pair of 

 vertical veno-pericardiac muscles, a pair for each segment tra- 

 versed by the pericardium. When these muscles contract the 

 cavities of the respective sinuses are enlarged and blood rushes 

 in ; they must therefore play a considerable part in the 

 distribution of the blood through the system. 



The pulmonary sacs, of which there are four pairs, communicate 

 with the outer air by slit-like stigmata placed obliquely on 

 the sternal plates of segments 9 to 12 (Fig. 519). Each of them 

 contains from 130 to 150 lamellae or leaves, which constitute 

 the so-called lung-books (p. 801). The leaves are borne on a 

 definite axis and part of the edge of each leaf adheres to the 

 wall of the respiratory chamber, only a small part of the edge 

 being free. Each leaf is hollow and the blood to be oxygenated 

 flows within the thin, slit-like cavity only separated from the 

 air by the membranous walls. 



The nervous system of Scorpio consists of a supra-oesophageal 

 ganglion which supplies the median and marginal eyes and a 

 very thickened and concentrated ring which embraces the oeso- 

 phagus. This ring, with which is aggregated a number of post- 

 oral ganglia, gives off paired nerves to the appendages of the 



