CIKCULATOKY SYSTEM OF SCORPION. 375 



aorta, descending suddenly into the thorax, and dividing immediately 

 behind the brain into a number of large vessels, that supply the head 

 and the locomotive organs. The posterior of these form a vascular 

 collar around the oesophagus, which gives origin to the great arterial 

 trunk, or supra-ganglionic vessel, whereby the blood is conveyed to the 

 posterior part of the body, as in the Myriapoda (vide 746). This 

 vessel passes beneath the transverse arch of the thorax, with which it 

 is slightly connected by fibrous tissue, and then runs backwards, gradu- 

 ally diminishing in size, until it reaches the terminal ganglia of the 

 tail, where it divides into branches that accompany the nerves. In 

 addition to the above arrangement, Mr. Newport has discovered a fibrous 

 structure, from which are given off two pairs of vessels, to be distributed 

 to the first pair of branchial organs, as also a little vessel which, passing 

 backwards, anastomoses with the spinal artery, to form the sub-spinal 

 vessel. This latter takes its course beneath the chain of nervous ganglia, 

 communicates directly by means of short branches with the supra- 

 ganglionic artery, and, at intervals, gives off from its under surface 

 large vessels, which, uniting together, convey the blood which has 

 circulated in the abdominal segments directly to the branchiae, whence 

 it is returned to the heart by a great number of slender canals, which, 

 emanating from the posterior aspect of each branchial organ, unite to 

 form larger trunks, that run along the walls of the segments, to pour 

 their contents into the valvular orifices situated upon the dorsal aspect 

 of the heart. 



(966.) The heart of the Scorpion* is a strong muscular organ ex- 

 tending along the middle of the back, from its continuation with the 

 great caudal artery in the last segment of the abdomen to the com- 

 mencement of the aorta. In the dorsal part of its course the heart is 

 divided into eight separate chambers, which are wider and stronger in 

 proportion to their length than in the highest of the Myriapoda. They 

 are more muscular and compact in proportion to the greater quantity of 

 blood to be transmitted through them, and the force with which it is 

 necessarily propelled. The form of each chamber is somewhat heart- 

 shaped, being slightly contracted in its middle portion and enlarged at 

 its posterior. Each chamber has two. auricular openings for the passage 

 of the blood, placed very close to the median line of the heart on its 

 dorsal surface ; and it gives off at its inferior lateral angles a pair of 

 large arterial vessels (the systemic arteries), which distribute the blood 

 downwards to the viscera and to the dorsal and lateral surfaces of the 

 body. 



(967.) Each chamber is provided at its sides, as in the Myriapoda 



and Insects, with two sets of muscles, the alee cordis. The anterior 



and larger pair of muscles are attached to the anterior part of each 



segment, and pass diagonally forwards, and the posterior (the proper 



* Newport, Phil. Trans. 1843. 



