. 

 THE ARTHROPODA 31,. 



The heart itself, lying close to the dorsal surface of the midline, 

 constricts when filled with blood. This constriction sends blood pos- 

 teriorly through the dorsal abdominal artery, which lies on the dorsal 

 surface of the intestinal tract, and through a short branch known as 

 the sternal artery, which passes downward crossing the intestinal tract. 

 The blood is also thus sent to the ventral thoracic artery anteriorly, and 

 posteriorly to the ventral part of the body through the abdominal artery. 



The arteries passing out of the 

 anterior portion of the heart are the 

 ophthalmic, supplying the stomach. 

 oesophagus, and head, and the two 

 antennary, carrying blood to the 

 stomach, antennae, excretory or- 

 gans, and the various other tissues 

 of the head. The two hepatic 

 arteries lead to the digestive glands. 

 When the blood is forced 

 Fig - 20 ,i through the arterial system, the 



Astacus fluviatilis. The heart A, From ' 



above; B, from below; C, from the left side: heart naturally COllapSCS, and the 



a.a., Antennary artery ; a.c., alae cordis, or , , t , . , , . .. 



fibrous bands connecting the heart with the blood which has been Sent OUt forCCS 



the blood which is then present in 

 the arteries to be sent forward 



abdominal artery ; st.a., sternal artery in B through the glands. ThcSC glands 

 cut off close to its origin. (From Dougherty 



after Huxley.) act similarly to the lungs in the 



higher forms of animals, aerating the blood and sending it to the large 

 open place around the heart known as the pericardial sinus. The heart 

 itself has two openings on both dorsal and ventral surfaces, and one on 

 each side. The heart muscles, after constriction, again assume their nor- 

 mal state when the blood in the pericardial sinus seeps through the six 

 heart openings, filling the cavity. Each of the openings possesses a valve 

 which prevents the blood from passing out, except through the arterial 

 channels. 



It is interesting to note that this method is just the reverse of that 

 occurring in the fishes where the blood passes through the heart first 

 and thence to the gills, while in the crayfish it is the returned blood that 

 passes .through the gills before reaching the heart. 



Unless colored matter of some kind is injected into the circulatory 

 system the student will probably have some difficulty in finding either 

 heart or arteries. 



Valves are present in all the arteries at the point of connection with 

 the heart, and blood passes into numerous capillaries and thence into the 

 open spaces between the tissues, until it reaches the external sinuses, 

 from which it enters the gill channels, to pass into the gill filaments 

 where oxygen from the water in the branchial chambers is- exchanged for 

 the carbonic acid that is held in solution in the blood. From here it 



