THE DEVELOPMENT OP SYMBRANCHUS MARMOEATUS. 
the body-segments from the dorsal aorta (PI. 4, fig. 27 ), while 
segmental veins convey venous blood from the muscles into 
the blood sinus which bathes the excretory organs. The 
inter-renal vein is double in the neighbourhood of the urinary 
bladder, this structure lying between the two branches; the 
right blood-vessel is always the larger. 
The free portion of the left posterior cardinal (compare 
Text-fig. 3a) is reduced in size at this stage ; eventually it 
atrophies completely, the blood of the inter-renal vein being 
conveyed to the heart by the right posterior cardinal. The 
vitelline vein, instead ot opening ventrally into the sinus 
venosus, has its opening shifted well to the left so that it opens 
into the ductus Cuvieri symmetrically with the large right 
posterior cardinal (Text-fig. 3b). It has been shown in the 
earlier stages that the liver is practically a part of the yolk 
as regards its circulation. It has also been shown that the 
caudal vein is continued forwards as an inter-renal (which, 
however, is a paired vessel in the neighbourhood of the urinary 
bladder) and a subintestinal vein. This division of the 
caudal vein takes place at the anterior end of the urinary 
bladder. In the earlier stages the subintestinal vein breaks 
up in the yolk into a number of capillaries which unite to form 
a large vitelline vein. However, as the creature grows, and 
as more and more of the yolk-mass becomes included in the 
normal contour of the animal, the subintestinal vein never 
loses its individuality, so to speak, but can be traced forwards 
through the liver, becoming the hepatic vein. This vein 
gradually increases in diameter as the pericardium is reached^ 
because it receives blood from the yolk and liver. As the 
creature grows and as the liver elongates blood communica- 
tions between kidney and liver are also set up. This is very 
striking in Stage 30, when the subintestinal vein gives off a 
large branch, which, running ventral to the alimentary 
canal, enters the posterior end of the liver and there 
breaks up into capillaries. Later on more blood passes into 
the liver from the hinder parts of the creature, this blood 
eventually finding its way into the anterior parts of the sub- 
