PHYLUM ARTHROPOD A 283 



enters the heart from the surrounding sinus through three pairs 

 of valvular ostia. Rhythmical contractions then force it for- 

 ward, backward, and downward. 



(i) The ophthalmic artery (Fig. 202, 34) supplies part of the 

 stomach, the oesophagus, and head> 



(2, 3) The two antennary arteries (35) carry blood to the 

 stomach, antennae, excretory organs, and other cephalic tissues. 



(4, 5) The two hepatic arteries (36) lead to the digestive 

 glands. 



(6) The dorsal abdominal artery (31) supplies the intestine and 

 surrounding tissues. 



(7) The sternal artery (30) divides into a ventral thoracic and a 

 ventral abdominal artery which carry blood to the appendages 

 and other ventral organs. 



SINUSES. The blood passes from the arteries into spaces 

 lying in the midst of the tissues, called sinuses. The heart lies 

 in the pericardial sinus. The thorax contains a large ventral 

 blood space, the sternal sinus T and a number of branchio-cardiac 

 canals extending from the bases of the gills to the pericardial 

 sinus. A perivisceral sinus surrounds the alimentary canal in the 

 cephalothorax. 



THE BLOOD FLOW. The heart, by means of the rhythmical 

 contractions, forces the blood through the arteries to all parts 

 of the body. Valves are present in every artery where it leaves 

 the heart; they prevent the blood from flowing back. The 

 finest branches of these arteries, the capillaries, open into spaces 

 between the tissues, and the blood eventually reaches the sternal 

 sinus. From here it passes into the efferent channels of the gills 

 and into the gill filaments, where the carbonic acid in solution is 

 exchanged for oxygen from the water in the branchial chambers. 

 It then returns by way of the afferent gill channels, passes into 

 the branchio-cardiac sinuses, thence to the pericardial sinus, and 

 finally through the ostia into the heart. The valves of the ostia 

 allow the blood to enter the heart, but prevent it from flowing 

 back into the pericardial sinus. 



