274 TEXTBOOK OF ZOOLOGY 



and blood is drawn in from the surrounding pericardial sinus in 

 which the aerated blood accumulates. When the heart contracts, 

 blood is forced into the anterior region of the body through the single 

 anterior median artery, paired antennary, and paired hepatic arteries 

 all of which arise from the anterior end of the heart. The large dorsal 

 ahdominal artery extends from the posterior tip of the heart pos- 

 teriorly through the abdomen just dorsal to the intestine. It sup- 

 plies the intestine and muscles of the body wall. The sternal artery 

 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 ahdominal 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 

 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. 



The excretory system consists principally of a pair of large 

 bodies located in the ventrolateral portion of the head. These are 

 richly supplied with blood and draw the nitrogenous wastes and 

 excess water from the blood to deliver them externally through 

 excretory pores located in the coxopodites of the antennae. 



The nervous system is of the same structural plan as that of the 

 earthworm, which is a modified "ladder type.'' The two longi- 

 tudinal cords have come together in the ventral line and run the entire 

 length of the body to form a ventral nerve cord with ganglia. This 

 arrangement constitutes the central nervous system. The ganglia 

 of the anterior three segments are fused into the "brain" or supra- 

 esophageal ganglion which is located anterior to the esophagus and is 

 joined to the cord by two circumesophageal commissures or connec- 

 tives, one passing on each side of the esophagus. From this dorsal 

 ganglionic mass, nerves pass to the eyes, antennae, and antennules. 

 The most anterior portion of the ventral cord receives these com- 



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