TRACHEATA : 303 
canals to the pericardium, and so by the ostia into the con- 
tractile heart. The blood has a bluish tinge, due to the pre- 
sence of haemocyanin, and contains numerous oval corpuscles. 
Fic. 224.—Diagram of cir- 
culatory system of Limulus 
polyphemus. From Leuck- 
art, after Milne-Edwards. 
\ \Weezz<-------9 
(a 2 10 
— 
SS 
\ 
ee 8 
/ 13 
1. Oesophagus. 10. Ventral artery surrounding ventral 
2. Heart with 8 pairs of ostia. nerve cord and giving off branches 
3. Aortic arch, the two branches of to mesosomatic limbs and gills. 
which unite into a ring surround- 11. Longitudinal lateral vein giving 
ing the oesophagus and mouth. branches to gills, the blood is re- 
4. Frontal artery. turned from gills to pericardium by 
5. Marginal artery. the vessels marked (22) in previous 
6. Collateral artery, running parallel figure. 
with heart, and supplied by 7 12. Cutaneous nerves which have no 
vessels from the heart. arterial sheath. 
Supra-anal artery. 13. Ventral nerve cord in ventral artery. 
Sub-anal artery. 
One of the 5 pairs of vessels going to 
the 5 thoracic limbs. 
am 
. Lateral eye. 
The gills, which are borne upon the posterior face of the 
five posterior pairs of appendages, consist of a number (150) of 
very delicate plates, each composed of two thin membranes, in 
the space between which the blood circulates. 
The plates lie 
parallel to one another like the leaves of a book, and the water 
circulates between them. 
A large gland, in its natural state of a brick-red colour, is 
situated at the base of the leg-like appendages. 
Tt consists of 
a longitudinal portion, from which four lobes are given off, 
