192 ARTHROPODA 



the pericardium. Locate the eight pairs of ostia, which are 

 transverse, slitlike valves opening from the pericardium into 

 the heart. Note the median and the two lateral cardiac 

 nerves lying on the pericardium. From the anterior end of 

 the heart extend the median frontal artery and the two lateral 

 aortic arches. The former extends forward and downward 

 and branches into two marginal arteries. The aortic arches 

 pass down on either side of the proventriculus and follow 

 the esophagus to the nerve collar. Here the vascular system 

 widens to enclose the collar, then extends posteriorly, en- 

 closing the ventral cord (vascular ring and ventral artery). 

 Branches from the ventral artery supply the telson. 



Along the sides of the anterior half of the heart may be 

 found four pairs of lateral arteries. These are valved tubes 

 leaving the heat just beneath the first four pairs of ostia. 

 The lateral arteries on either side lead into a collateral ar- 

 tery extending in a posterior direction. Each of the col- 

 lateral arteries gives off branches going to the muscles and 

 the hemal surfaces of the body, and other branches (directed 

 toward the median line) to the digestive tube. The two col- 

 lateral arteries unite just behind the heart to form the 

 superior abdominal artery. Branches of this artery anas- 

 tomose with branches of the ventral aorta. 



Five pairs of branchiocardiac canals or sinuses bring 

 blood from the gills to the pericardial sinus. The first of 

 these receives blood from the operculum and the first gill; 

 the remaining four receive blood from gills 2, 3, 4, and 5 

 respectively. 



The remaining portions of the circulatory system are not 

 so easily seen, but will be mentioned for the sake of com- 

 pleteness. The vascular ring supplies blood to the appen- 

 dages of the cephalothorax. The ventral artery supplies the 

 gills. Blood from all parts of the body collects in a pair of 

 longitudinal sinuses which lead to the gills. 



The Digestive System. — Remove the heart and such gonad 

 and liver tissues as lie in the way. Observe the large sac- 

 like proventriculus or stomach, extending into the anterior 



