190 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



blood, and giving off in regular metameric fashion a 

 pair of arterial vessels. Anteriorly the cardiac 

 trunk is continuous with a vessel which lies on the 

 upper surface of the ventrally placed nerve cord. 



In the Hexapoda the Cardiac tube is confined 

 to the abdominal region of the body, and there is a 

 smaller number of separate chambers. The anterior 

 tube is known as the aorta ; its further ramifications 

 are not known. In them and in the Myriopoda 

 the pericardiac sinus has connected with it several 

 pairs of ordinarily fan-shaped muscles (alse cordis) ; 

 as these are not directly attached to the cardiac tube, 

 they cannot, as has been sometimes supposed, have 

 any function in the way of dilating the heart ; it is 

 probable, however, that they enlarge the extent of the 

 pericardiac sinus, and thereby assist or accelerate the 

 flow of blood into it. The inner lining of the heart is 

 elastic, and the outer muscular coat does not contract 

 simultaneously, but from behind forwards (Lowne). 



The blood-vascular system of the Araclmida, as 

 represented by Limulus and Scorpio, is more com- 

 plete than that of any other Arthropod ; fine vessels 

 given off from the arteries form a true capillary 

 system, and the veins are definite and distinct. The 

 heart is elongated, and consists of eight chambers, 

 each provided with a pair of apertures guarded by 

 valves ; it is continued forwards into an anterior, and, 

 in the scorpion, backwards into a posterior aorta. In 

 the scorpion each cardiac chamber gives off an artery 

 on either side, and several pairs are given off from the 

 posterior aorta. Anteriorly, the aorta forms a collar 

 round the cerebral nerve-mass, and is continued into 

 a ventral artery which lies above the ventral nerve- 

 cord ; this artery is intimately connected with the 

 nerve-chain in the scorpion, and in Limulus it absor 

 lutely surrounds it. 



In the spiders and other Arachnids the number of 



