SEA-FISHERIES LABORATORY. 407 



above the hind-gut, but not in the median line. As it 

 passes backward it gradually crosses over to the right 

 side. In each of the second, third, fourth and fifth 

 abdominal segments a pair of arteries is given off to the 

 appendages. At the posterior end of the fifth segment 

 the aorta divides into two branches. The right branch 

 follows the course of the aorta. The left branch crosses 

 over the hind-gut and is continued down the left side of 

 the hind-gut parallel to the right branch. Both 

 branches pass into the telson, where they break up into 

 fine branches. Throughout the abdomen small branches 

 are given off from the posterior aorta to the hind-gut, to 

 the abdominal muscles and the muscles of the abdominal 

 appendages. 



Male. There are only two pairs of arteries given 

 off from the posterior aorta. These are in the first and 

 second abdominal segments and supply the two pairs of 

 appendages. As the aorta passes backward it crosses over 

 to the left side, and in the fifth segment it bifurcates, the 

 right branch crossing over the hind-gut. As in the 

 female, there are also innumerable small arteries given off 

 to the hind-gut and to the abdominal muscles. 



Descending artery- (PL IX, fig. 56, d.art.). This 

 leaves the heart at the posterior end close to the 

 origin of the posterior aorta. It is an extremely 

 wide vessel which passes almost directly downward 

 on the right side of the hind-gut until it nearly 

 reaches the anterior part of the " sella turcica," where it 

 turns suddenly forward. It continues to pass downward 

 and forward, and between the muscles of the fourth and 

 fifth thoracic appendages it passes through the foramen 



* The term sternal artery is generally given to this artery, as well 

 as to its continuation along the ventral side of the thorax. I have, 

 however, thought it more fitting to apply the term " sternal artery " 

 only to the ventral portion . 



