The Dissection of the Fish 27 
Next it flows into the thick-walled ventricle, whence by the 
rhythmical construction of its walls it is forced into an arterial 
bulb which lies at the base of the ventral aorta, which carries 
it on to the gills. After passing through the fine gill-filaments, 
it is returned to the dorsal aorta, a large blood-vessel which ex- 
tends along the lower surface of the back-bone, giving out branches 
from time to time. 
The kidneys in fishes constitute an irregular mass under the 
back-bone posteriorly. They discharge their secretions through 
the ureter to a small urinary bladder, and thence into the uro- 
genital sinus, a small opening behind the anus. Into the same 
sinus are discharged the reproductive cells in both sexes. 
In the female sunfish the ovaries consist of two granular 
masses of yellowish tissue lying just below and behind the swim- 
bladder. In the spring they fill much of the body cavity and 
the many little eggs can be plainly seen. When mature they 
are discharged through the oviduct to the urogenital sinus. In 
some fishes there is no special oviduct and the eggs pass into the 
abdominal cavity before exclusion. 
In the male the reproductive organs have the same position 
as the ovaries in the female. They are, however, much smaller 
in size and paler in color, while the minute spermatozoa appear 
milky rather than granular on casual examination. <A vas defe- 
rens leads from each of these organs into the urogenital sinus. 
The lancelets, lampreys, and hagfishes possess no genital 
ducts. In the former the germ cells are shed into the atrial 
cavity, and from there find their way to the exterior either 
through the mouth or the atrial pore; in the latter they are shed 
directly into the body cavity, from which they escape through 
the abdominal pores. In the sharks and skates the Wolffian 
duct in the male, in addition to its function as an excretory duct, 
serves also as a passage for the sperm, the testes having a direct 
connection with the kidneys. In these forms there is a pair 
of Millerian ducts which serve as oviducts in the females; they 
extend the length of the body cavity, and at their anterior end 
have an opening which receives the eggs which have escaped 
from the ovary into the body cavity. In some bony fishes as 
the eels and female salmon the germ cells are shed into the body 
cavity and escape through genital pores, which, however, may 
