TRACHEATA 



393 



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. 



Fig. 224. — Diagram of cir- 

 culatory system of I/imulus 

 polyphemus. From Leuck- 

 art, after Milne-Edwards. 



Oesophagus. 



Heart -with 8 pairs of ostia. 



Aortic arch, the two tranches of 

 which unite into a ring surround- 

 ing the oesophagus and mouth. 



Frontal artery. 



Marginal artery. 



Collateral artery, running parallel 

 with heart, and supplied by 7 

 vessels from the heart. 



Supra-anal artery. 



Sub-anal artery. 



One of the 5 pairs of vessels going to 

 the 5 thoracic limbs. 



10. Ventral artery surrounding ventral 



nerve cord and giving off branches 

 to mesosomatic limbs and gills. 



11. Longitudinal lateral vein giving 



branches to gills, the blood is re- 

 turned from gills to pericardium by 

 the vessels marked (22) in previous 

 figure. 



12. Cutaneous nerves which have no 



arterial sheath. 



13. Ventral nerve cord in ventral artery. 



14. 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. It consists of 

 a longitudinal portion, from which four lobes are given off. 



