212 PHYLUM CHORD AT A 



duct, or sinus, called the Cuvierian duct. The two Cuvierian ducts 

 lie along the anterior end of the abdominal cavity, parallel with 

 each other and just in front of the liver, between the sinus 

 venosus and the head kidney. Inasmuch as the head kidney is 

 dorsal in position and the sinus venosus ventral, the Cuvierian 

 ducts have an almost vertical position in the body. Trace these 

 vessels from the sinus venosus dorsally ; they will be found just 

 behind the posterior gill arch. The dorsal ends of these two 

 ducts are connected by a horizontal sinus which lies just beneath 

 the spinal column. 



Two pairs of prominent veins, the jugulars, or anterior cardi- 

 nals, and the posterior cardinals, bring blood from the anterior 

 and the posterior portions of the body, respectively, to the hori- 

 zontal sinus of the Cuvierian ducts. Just before joining the sinus 

 the two jugulars unite and form a median vein ; the two posterior 

 cardinals also unite and form a median vein. 



The two posterior cardinals are large and prominent veins and 

 lie just beneath the vertebral column, partly embedded in the 

 kidneys ; the dorsal aorta lies between them. The left cardinal 

 is much larger than the right, and much longer, and begins its 

 course at the posterior end of the body as the caudal vein ; this 

 vein lies in the ventral arches of the spinal column and receives 

 the paired spinal veins. 



In the trunk region it receives the left spinal and renal veins. 

 The right cardinal begins its course in the hinder part of the body 

 cavity and receives the right spinal and renal veins. Find the 

 cardinals in the mass of the kidneys and trace them forward to 

 the median cardinal and the Cuvierian ducts. 



Find the median jugular vein and trace it and the two jugulars 

 and their branches as far as possible. 



Entering the horizontal sinus are three additional veins. Two 

 of these are the small, paired subclavian veins, which bring blood 

 from the pectoral fins; the third is the intestinal vein, which 

 brings blood from the stomach, intestine, and genital organs. 

 Follow the course of these veins and their branches. 



Exercise IS. Draw a diagram of the venous system, so far as observed. 



