BLOOD-VASCULAR SYSTEM OF THE LORICATI 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 Opfrodon is that shown in fig. 12. 
The vessel designated as the rzght ventral vein (Pl. I, fig. 
12; R.Ven.V.) is a deéper 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 77g? and a left ventral jin 
dem (El. 11, ig. 12; R. and U-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 (P\. Il, 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 /eft ventral fin vein (fig. 12; 
