URINARY ORGANS. lv 



also that in the highly-organized sharks and rays, instead of these appendages 

 we find a spiral valve existing in the intestines, as already described. 



URINARY ORGANS. 



The urinary organs are composed of the kidneys as a secreting or 

 excreting apparatus, the ureters, the bladder, and the urethra, the two last 

 being absent in some fishes. The kidneys, according to Vogt and some 

 others, appear to be composed of the Wolfian bodies, structures persistent 

 among fishes, but which are not permanent among other vertebrate animals, 

 whereas' other authors deny this. Whatever they may be considered, still 

 two kidneys, generally well-developed, are always present among fishes, 

 except, perhaps, in the amphioxus. These organs, as a rule, are distinctly 

 separated anteriorly one from the other, but sometimes more or less 

 united posteriorly : they are placed close to the vertebral column, but 

 separated from the abdominal cavity and the intestines by the peritoneum. 

 To this rule there are exceptions, as among teleosteans, which possess an air- 

 bladder where it likewise is outside the peritoneum, and is often more or less 

 adherent to the under surface of the kidneys. 



The size as well as the form of the kidneys is subject to great variation : 

 in some teleosteans they are compact and restricted to the forepart of the 

 abdominal cavity, but in others they extend far forward to the base of the 

 skull and posteriorly, as seen in the eel, &c, to between the muscles of the 

 caudal region, while their surface may be smooth or lobulated. The ureters, 

 which receive the urine from the urinary tubes, are generally two in number, 

 rarely three, while as many as five from either kidney, each of which 

 separately opens into the urinary bladder, have been found in sticklebacks. 

 These ureters are usually placed along the inner edge of the kidneys, and 

 may be continued to their outlet, or they may unite in an urinary bladder 

 which opens behind the vent by a short urethra sometimes confluent with, 

 sometimes distinct from (and is then behind) the genital opening, while 

 occasionally it is situated on a papilla. Among the Plagiostomes the ureters 

 are short, and each forms a dilatation which anastomosing with its fellow, 

 ends in a single urethra, and having received the vasa deferentia in the 

 male opens into the cloaca behind the end of the rectum. Variations in the 

 positions of the various parts are seen among the Ganoids and Dipnoids. 

 Among the Cyclostomes, as the lampreys, the kidneys are in the form of an 

 elongated gland, with detached portions, the ureters coalescing prior to 

 terminating in the urogenital papilla. In the hag a long duct extends 

 through the abdominal cavity, sending off short transverse branches, each 

 of which ends in a blind sac, where a ganglion that secretes the urine is 



