KIDNEYS. 



Ill 



The kidneys are not divided into meso- and meta-nephros. 

 They are placed along the dorsal side of the body cavity for the 

 middle of its length, being absent at the anterior and posterior 

 ends. The longitudinal ducts (pronephric ducts) extend along 

 their whole length and join together posteriorly in Petromyzon 

 to open by a single opening into the 

 urogenital sinus. The urogenital sinus 

 which also receives the two genital pores 

 opens at the end of a papilla just be- 

 hind the anus into a depression of the 

 skin into which the anus also opens. 

 In Ammocoetes the kidney ducts open 

 into the hind end of the intestine 

 (cloaca). The separation of the uro- 

 genital sinus and the formation of the 

 genital pores takes place just before 

 the metamorphosis. 



In Myxinidae * there is a shallow 

 cloaca which receives the opening of the 

 intestine in front, the wide genital pores 

 (single in Myxine) on the dorsal side of 

 the anus, and the two urinary ducts, 

 opening close together at the end of a 

 papilla behind. 



In Myxine the kidneys have an ex- 

 ceedingly simple structure. The longi- 

 tudinal ducts give off at segmental 

 intervals short lateral tubes which open 

 into large malpighian bodies. The 

 glomeruli are multipolar, i.e., are con- 

 nected at several places with the wall 

 of the capsule. In Petromyzon the 

 structure is very similar but more 

 complicated, though the malpigliian 

 capsules of successive tubules are separate, the vascular tissue 

 of the glomeruli is continuous. 



The tubules of the kidney do not open into the body cavity 

 by nephrostomata. 



Fig. 58. — Portions of the kidney 

 of Myxine (after J. Muller). 

 B represents a portion of 

 A highly magnified ; a pro- 

 nephric (longitudinal) duct ; 

 b kidney tubule ; c glome- 

 rulus ; d afferent, e efferent 

 artery. 



* R. H. Burne, " Porus genitatis in Myxinidae,''^ Journ, Linn. Soc, 26, 

 1898, p. 487. 



