546 PISCES——FISHES., 
a jugular vein ; (4) from the liver by a hepatic sinus, which runs from 
one precaval sinus to the other like the string of the bow; (c) froma 
large posterior cardinal sinus (between the reproductive organs) by 
a cardinal vein on each side; (@) from the hind-fin by an epigastric, 
with which brachials from the fore-limb unite anteriorly. The great 
cardinal sinus receives blood from the hind-limbs, the kidneys, and 
other posterior parts. 
Blood asses zo the liver (a) from the cceliac artery, and (4) by 
portal veins from the intestine (the hepatic portal system) ; blood /eaves 
the liver by hepatic veins which enter 
the hepatic sinus. 
Blood fasses znto the kidneys (a) 
from the renal arteries, and (4) by 
renal portal veins from the caudal, 
pelvic, and lumbar regions (the renal 
portal system); blood eaves the 
kidneys by posterior cardinal veins, 
which enter the cardinal sinus. 
Into the precaval sinus there also 
opens the lymphatic trunk. 
The heart lies in a_ pericardial 
cavity, which is connected with the 
abdominal cavity by two fine canals, 
and is an anterior part of the ccelom. 
The blood contains, as usual, red 
blood corpuscles and leucocytes. 
The dark red spleen lies in the 
curve of the stomach. The red 
thyroid gland lies just in front of 
the anterior end of the ventral aorta. 
The thymus is represented by a 
whitish body dorsal to each of the 
first four gill-clefts. | Each begins as 
a patch of endoderm, and this is 
invaded by migratory mesenchyme 
cells which multiply as lymphocytes. 
Fic. 288.—Urogenital organs 
of male skate. E : 
is Teter why, wolaiagiiiaY wt, xcretory and reproductive 
vas. deferens + °K idney § aoe systems.—The dark red kid- 
ee tec ainey CA, eloace, neys lie far back on each 
side of the vertebral column. 
They are developed from the hind part of the mesonephros. 
Several tubes from each kidney combine to form a ureter. 
The two ureters of the male open into the urogenital 
sinus, whence the waste products pass out by the cloaca; 
in the female they open into little bladders,—the dilated 
ends of the Wolffian ducts,—and thence by a common 
aperture into the cloaca. _ 
