CHAPTEE XT. 



URINARY ORGANS. 



In BrancMostoma no urinary organs liave been found. 



In Myxinoids these organs are of a very primitive structure : 

 they consist of a pair of ducts, extending from the urogenital 

 porus through the abdominal cavity. Each duct sends off at 

 regular intervals from its outer side a short wide branch (the 

 uriniferous tube), which communicates by a narrow opening 

 with a blind sac. At the bottom of this sac there is a small 

 vaso-ganglion (Malpighian corpuscle), by which the urine is 

 secreted. 



In the Lampreys the kidneys form a continuous gland- 

 like body, with irregular detached small portions. The 

 ureters coalesce before they terminate ia the urogenital 

 papiEa. 



In Chondropterygians the kidneys occupy the posterior 

 half or two-thirds of the back of the abdominal cavity, without 

 the sac of the peritoneum (as in all fishes) which forms a firm 

 tendinous horizontal septum. The kidneys of the two sides 

 are never confluent, and generally show a convoluted or 

 lobulated surface. The ureters are short ; each is dilated into 

 a pouch, and communicating with its fellow terminates by 

 a single urethra (which also receives the vasa deferentia) 

 behind the end of the rectum in the large common cloaca. 



In Ganoids the kidneys occupy a similar position as in 

 Chondropterygians, but these fishes differ considerably with 

 regard to the termination and the arrangement of the ends of 



