riorly each is a narrow band extending forward, then out, to 

 the duct of Cuvier. In the male, the testis connects by nu- 

 merous long efferent ducts through the mesorchium with 

 the anteromiddle part of the kidney and through this with 

 the nephric duct. The female has a large funnel opening 

 into the nephric duct just where that duct turns ventrally to 

 form the urogenital sinus (much like Aapenser or Polyodon). 



The connection between oviduct and nephric duct is of 

 interest. The oviduct ends blindly in the connective tissue 

 sheath of the nephric duct and may bulge into it. Just an- 

 terior to this tip is a small slit or pore connecting these 

 ducts. Amia has a urinary bladder projecting forward from 

 each nephric duct just before that duct enters the urogenital 

 sinus. This sinus opens into a small fold a short distance be- 

 hind the anus. 



Lepisosteus has a much more elongate but similar kidney 

 (Figure 10-46). There is no marginal canal for the connec- 

 tion with the testis. As in Anna, the nephric duct extends 

 along the lateral margin of the kidney. Posteriorly the 

 nephric duct increases in diameter and has elastic walls 

 serving as a urinary bladder. The oviduct of the female 

 opens into the anterior end of this expanded part. The 

 nephric ducts form a midline urinary sinus which opens to 

 the outside behind the anus. 



EMBRYOLOGiCAL DEVELOPMENT In its development, Lepisos- 

 teus is much like Acipenser. The fourth to seventh metotic 

 segments develop funnels. The most anterior one opens into 

 the coelom, the others into their separate nephrocoels, which 

 in turn connect with the coelom. What appear to be acces- 

 sory funnels so develop that the tubules are connected with 

 the coelom not only through the nephrocoel but also through 

 additional funnels (true peritoneal funnels). A single median 

 glomerulus develops from the several pairs of glomeruli, and 

 the several nephrocoels fuse to form a single cavity for the 

 several tubules. In a 40-mm larva there were an anterior 

 coelomic funnel, three funnels associated with the glomeru- 

 lar capsule (space around glomerulus), and a peritoneal 

 funnel associated with the tubules from this capsule. 



The pronephros is still present, although degenerating, at 

 70 mm; soon thereafter it has degenerated and is largely 

 replaced by lymphoid tissue. 



In Amia the pronephros has three nephrocoels at the 

 beginning; the anterior one of these opens into the coelom. 

 A compound midline glomus forms and the two posterior 

 tubules fuse. The pronephric duct is somewhat more con- 

 voluted than in Lepisosteus. The nephric duct appears to be 

 continued back from the pronephric region as a ridge formed 

 from the intermediate plate mesoderm and then separated 

 as a cord of cells. 



The mesonephros develops as in other forms (Figure 10- 

 23). In the early stages, secondary and tertiary tubules open 

 into the nephric duct. Ciliated peritoneal funnels are pre- 

 sent up to about 100 mm; however, many tubules lack fun- 

 nels, while others have two. Some funnels end blindly in the 



kidney substance. With the development of the oviduct, 

 some funnels open inside of this tube in both Amia and 

 Lepisosteus, as in Acipenser and Polypterus. 



General observations The actinopterygians, including 

 Polypterus, agree in having a multifunnel pronephros which is 

 associated with a median anterior glomus. The number of 

 tubules associated with the glomus tends to be reduced 

 both by fusion and degeneration, so that a single anterior 

 tubule results in Polypterus, Amia, or the teleosts. The pro- 

 nephros generally degenerates, although the whole or its 

 most posterior part may be retained as a lymphoid strand. 

 Occasionally a functional pronephric tubule is retained in 

 teleosts and in Polyodon. In the teleosts, mesonephric peri- 

 toneal fLinnels are lacking even in the early stages. Such 

 funnels are lacking in the adults of all actinopterygians. 



midiine glomerulus 



pronephric convolution 



— 5th somite 



pronephric funnel 



chromaffin bodies 



Stannius bodies 



glomeruloe 



nephric duct 



points of entrance of nephrons 



funnels 



Figure 10-23. Ventral view of kidney of 22-mm Amio showing 

 pronephric elements, nephric duct, positions of mesonephric glomeruloe 

 and funnels, and Stannius and chromofTin bodies. (After Kindahl, 1938) 



THE EXCRETORY SYSTEM • 309 



