22 



as to closely invest them. The greater part of the blood 

 fiowing back through the aorta passes below the posterior 

 diverticula, since the latter lie nearer to the dorsal wall 

 of the aorta than to the ventral one, which is only slightly 

 separated from the mid-gut. The aorta is con- 

 tinued backwards above the rectum to the middle of 

 the last abdominal segment, where it splits into 

 two branches which turn downwards, one on each 

 side of the rectum, piercing the pericardium, and empty 

 their contents into the sternal sinus (PI. III., fig. 1, st. s.). 

 The anterior aorta (PI. III., hg. 1, a. ant.) is formed 

 by the sharp lateral constriction, and more gradual 

 lessening in diameter, of the anterior portion of the heart, 

 at the beginning of the first thoracic segment. A careful 

 examination failed to bring to light any valve. The 

 aorta passes gradually downwards, following closely the 

 dorsal wall of the stomach, till it reaches the level of the 

 first antennae. It then divides into two branches, the 

 upper one passing straight forward, above the eyes, to the 

 first antennae, etc., the lower one passing downwards close 

 to the wall of the stomach. After giving off a small 

 vessel which runs forwards and supplies the brain, the 

 lower branch receives a pair of small vessels from the 

 excretory organs (PI. III., fig. 1, exc. or.), and splits into 

 two large vessels, which sej^arate to enclose the 

 oesophagus and again unite, forming the pericesophageal 

 ring. After giving off a pair of branches to the maxilla? 

 and another pair to the maxillipeds, this single median 

 vessel opens into the sternal sinus. The upper branch of 

 the anterior aorta passes forward to the base of the first 

 antennte, to which it sends ofl' a pair of branches, one 

 running down the lower edge of each antenna. The main 

 artery then turns downwards, gives oif a pair of branches 

 to the second antennae, and, a little lower, divides into 



