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 toit. The collateral vessels unite behind, and 
are continued backwards as a single supra-anal vessel. Still 
Fic. 223.—Limulus polyphemus. Diagrammatic view of left half of body seen from 
the inside. From Leuckart, partly after Packard. 
1-5. The 5 pairs of thoracic limbs. 19. Bile ducts; the openings of two 
6. The operculun. into the intestine are seen. 
7-11. The gill-bearing mesosomatic limbs. 20. Liver. 
12. Mouth. 21. Heart, with eight venous ostia. 
13. Anus, 22. Efferent branchial veins leading into 
14. Spine or Telson. pericardium. 
15. Chilaria. 23. Frontal artery dividing at 
16. Oesophagus. 24. Into two marginal arteries. 
17. Muscular proventriculus, 25, Left aortic arch. 
18. Intestine. 26. Supra-anal artery; the sub-anal 
artery is shown ventral to this. 
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 rmg 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 
