392 



ZOOLOGY 



eight chambers, into the anterior end of each of which a pair 

 of ostia open (Fig. 223). The heart ends blindly behind, but 

 in front is continued into a truncus arteriosus ; from the base 

 of the latter an artery arises on each side, which passes, to- 

 gether with a vessel from each of the anterior chambers of 

 the heart, into a large collateral artery which runs outside the 

 heart, but parallel to it. The collateral vessels unite behind, and 

 are continued backwards as a single supra-anal vessel. Still 



5 15 



4 3 2 



Fig. 223. — Limulus polyphemus. Diagrammatic view of left half of body seen from 

 the inside, ^rom Leuckart, partly after Packard. 



The solid black structure beneath the alimentary canal, lying in a blood space, 

 is the nervous system. 



farther forward the truncus gives off two cerebral arteries, which 

 pass into a vascular ring which surrounds the oesophagus, and 

 encloses the nerve collar. From this ring a number of vessels 

 arise which supply the appendages, and it is continued backward 

 into a supraspinal vessel in which the ventral nerve cord lies. 

 The capillaries in which the arteries terminate open into 

 certain reservoirs ; passing from these the blood traverses the 

 gills, and is then returned by a system of branchiocardiac 



