28 The Dissection of the Fish 



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 Miillerian 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 



