162 ORGANIZATION OF THE HEAD v. 13 



13. Urinogenital system 



The blood of the elasmobranchs differs from that of all other verte- 

 brates in its very high content of urea. As measured by the depression 

 of the freezing-point the blood is isotonic with the surrounding sea 

 water (say, 3-5 per cent. NaCl); it may even be slightly hypertonic. 

 But there is far less salt in the blood than in the sea, in fact only 

 about 1-7 per cent. NaCl. Although the blood is nearly isotonic with 

 the sea its composition is therefore regulated (homeosmotic). This 

 arrangement is apparently a legacy of the fact that the ancestors of the 

 elasmobranchs were originally fresh- water animals (p. 187). The 

 return passage to the sea has been accomplished by the elasmobranchs 

 through the device of urea retention. The gill surfaces, in which alone 

 the blood comes into close contact with sea water, are not permeable 

 to urea, but this substance penetrates freely into the tissues, as it does 

 in other animals. Elasmobranch tissues if placed in sea water are 

 therefore in contact with a strongly hypertonic medium. They are so 

 habituated to the presence of urea that they are unable to function 

 unless it is present in a concentration that would be toxic to most 

 animals. 



This arrangement has presumably been responsible for the fact 

 that few of the elasmobranchs have returned to fresh water. In the 

 case of the saw-fish Pristis, which lives some hundreds of miles 

 up the Mississippi and various rivers in China, Smith found that a 

 considerable concentration of urea is still maintained in the blood, 

 thus further increasing the work that these fishes, like any fresh-water 

 animal, must do in order to maintain an osmotic concentration above 

 that of the surrounding water. One shark (Carcharhinus nicaraguensis) 

 and some rays, Trygon, also live in fresh water. 



In the ordinary marine elasmobranchs the high urea concentration 

 is maintained by the presence of a special urea-absorbing section of 

 the kidney tubules. The urinary apparatus is a mesonephros and 

 these fishes show a considerable specialization in that the urinary 

 functions of this organ are separated from its generative ones in the 

 male. The hinder part of the kidney (sometimes called opisthone- 

 phros, the term metanephros should be used only for the definitive 

 kidney of amniotes, which has a different method of development) 

 consists of a mass of tubules ending in very large glomeruli, and a 

 section of each tubule has the function of urea absorption. All the 

 tubules join to form a series of five urinary ducts and these enter a 

 urinary sinus, opening to the cloaca. The sinus can be compared 



