THE CRAYFISH AND ARTHROPODS IN GENERAL 203 



the antennary arteries. They lead directly to the digestive 

 glands. 



A single dorsal abdominal artery arises from the posterior 

 end of the heart. 



(6) The dorsal abdominal artery (31) is a median tube leading 

 backwards from the ventral part of the heart, and supplying the 

 dorsal region of the abdomen. It branches near its point of 

 origin, giving rise to the sternal artery (jo) ; this leads directly 

 downward, and, passing between the nerve cords connecting ths 

 fourth and fifth pairs of thoracic ganglia (see p. 205) divides into 

 two arteries. One of these, the -ventral thoracic artery, runs for- 

 ward beneath the nerve chain, and sends branches to the ventral 

 thoracic region, and to appendages III to XIII; the other, 

 the ventral abdominal artery (32), runs backward beneath the 

 nerve chain, and sends branches to the ventral abdominal region 

 and to the abdominal appendages. 



SINUSES. The blood passes from the arteries into spaces lying 

 in the midst of the tissues, called sinuses. The pericardial sinus 

 has already been mentioned. The thorax contains a large ventral 

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

 canals extending from the bases of the gills to the pericardial 

 sinus. A perimsceral sinus surrounds the alimentary canal in 

 the cephalothorax. 



BLOOD FLOW. The heart, by means of rhythmical contrac- 

 tions, 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 bran- 

 chio-cardiac sinuses, thence to the ptricardial sinus, and finally 

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



