URINAEY AND GENERATIVE ORGANS. 



147 



intestine is used ; this blood after passing through the capillaries of 

 the liver is returned to the heart by one or more veins which cor- 

 respond to the inferior vena cava and open into the sinus venosus 

 between the two ductus Cuvieri. Such capillary systems must be a 

 considerable hindrance to the circulation of the blood and explain 

 the development of the so-called accessory hearts on the caudal vein 

 of the eel and on the portal vein of Myxine. 



The urinary organs of Fishes (fig. 596) consist of paired kidneys 

 extending along the backbone from the head 

 to the end of the body cavity, and giving off D 



two ureters which unite into a common duct Vs 

 on which a bladder is usually developed. The 

 urinary bladder and its duct always lie be- 

 hind the intestinal canal. In most Teleo- 

 steans the efferent duct of the bladder opens 

 by a common orifice with the sexual opening, 

 or on a special papilla behind the sexual 

 opening. In the Plagiostomes and Dipnoi 

 on the other hand a cloaca is developed ; in 

 the former the ureters and the generative 

 ducts open into the dilated terminal part of 

 the intestine i.e., the cloaca behind the 

 rectum ; while in the latter the ureters open 

 into the cloaca separately on each side. 



Generative organs. Excepting in certain 

 forms, such as Serranus and Chrysophrys, 

 which are hermaphrodite (also some carps), 

 Fishes are of separate sexes ; the two sexes 



FIG. 596. Kidneys of Salmo 



often present more (Macropodus) or less fario (after Hyrti). JB, Md- 

 (Tinca, Cobitis] considerable sexual differ- ne ^ s ' ^uietajr,, bladder- 



' ' ^ like dilation; Ur, efferent 



ences. The male and female reproductive duct of bladder; D, ductus 

 organs (fig. 591) often resemble one another Cuvieri; Vs ' 8ubclavian vei - 

 so closely in form and position that it is necessary to investigate their 

 contents in order to distinguish the sex, especially as external sexual 

 differences are frequently absent. 



The ovaries are paired (in the Myxinoids the Squalidce, and certain 

 Teleosteans, as Perca, Blennius, Cobitis, they are unpaired) elongated 

 sacs, which lie ventral to the kidneys at the sides of the intestine and 

 the liver. The ova originate on the internal transversely folded 

 walls of the ovaries in closed follicles in which they receive a thick 

 egg-capsule (with pores and micropyle), and escape thence into the 



