CRAYFISH 



403 



is a large branch arising from the dorsal abdominal artery just after 

 it leaves the heart. It passes ventrally through the nerve cord and 

 divides into a posterior, ventral abdominal artery and an anterior, 

 ventral thoracic artery. These branches carry blood to the ventral 

 portions of the body. Besides the pericardial sinus already men- 

 tioned, there are others returning the blood to this one. The sternal 

 sinus is the main one, and it is located beneath the thorax. From it 



_ - Suproesophoqeal qanqlion 



Circumesophoqeal connective 



<^-Suhesopbageal qanqlion 

 )^yL Thoracic qanqWon 



f?/ng for st<^rm\ artery 



—PJ- abdominal gnngl/on 



Loteml nerve 



j:^^ Ventral ner\/e cord 



Tegmenta/ division 



Terminal qanqlion 



Fig. 163. — Dorsal view of the nervous sj'^stem of crayflsli. The subesophageal 

 ganglion merges with tlie anterior ilioracic ganglia. 



several branches lead into the gills. This provides for a course 

 through the gills. From them blood is collected by branchio-cardiac 

 canals and delivered to the pericardial sinus. A perivisceral sinus 

 surrounds most of the alimentary canal and collects the venous blood 

 from it. This kind of system is called the open type because of the 

 large irregular spaces or sinuses instead of an evenly constructed set 

 of veins which make a complete circuit of the course. 



