BLOOD-VASCULAR SYSTEM OF THE LORICATl 93 



4. Ventral Veins. 



These veins correspond to, and drain the region supplied by 

 the posterior part of the ventral artery: namely, the ventral or 

 pelvic fins, their muscles, and the ventral portion of the myotomes 

 forming the thoracic walls. Considerable variation is shown in 

 these veins, since they may arise as 2 rather large veins of equal 

 size or one small vein and one large one, but the most common 

 arrangement for Ophiodon is that shown in fig. 12. 



The vessel designated as the right ventral vein (PI. II, fig. 

 12; R.Ven.V.) is a deeper vessel than the ventral artery, and 

 terminates in the left hepatic sinus. This vein may be said to 

 have its source from 2 branches, a I'ight and a left ventral fin 

 vein (PI. II, fig. 12; R. and L.Ven.F.V.), which have their 

 origin in the right or left ventral fin ray canal. In these canals 

 the veins run behind the arteries, and receive a branch from 

 the center of each ray. Leaving the canal of the last rays each 

 of these veins crosses above the corresponding ventral ray 

 artery, and passes cephalad, for some little distance, between 

 the ventral or pelvic superficial adductor muscle and the ventral 

 myotomes. Then after uniting with its fellow, the combined 

 trunk continues cephalad as the right ventral vein or the main 

 ventral vein. Along its course this vein and its 2 branches re- 

 ceive numerous vessels. Soon after leaving the ventral fin 

 canal, the left ventral fin vein receives a posterior ventral vein, 

 which runs parallel with the corresponding artery. This branch 

 receives several ventral intercostal veins (PI. II, fig. 12 ; V.- 

 Intc.V.) from either side. In addition to receiving a ventral 

 intercostal vein from the septum between each alternate pair of 

 myotomes, each ventral fin vein receives several branches from 

 the superficial and profundus adductor muscles, and at least 2 

 branches, coming up between the pelvic bones from the super- 

 ficial and profundus abductor muscles. The right ventral vein 

 itself also receives at least 2 ventral intercostal veins from the 

 right side. In the specimen from which fig. 12 was drawn the 

 left ventral vein (L.Ven.V.) was a very short vessel, arising 

 from several ventral intercostal veins from the left side, but in 

 other specimens the left ventral vein was as large as the right, 

 and the vessel designated as the left ventral fin vein (fig. 12; 



