WATER REGULATION 



3^3 



forming a ven* dilute urine : 

 is rendered more watery by 

 by the kidney tubules. 

 Marine fishes are liable to 

 the opposite danger of the 

 loss of water. Bony fishes 

 counter this by drinking 

 sea water and excreting 

 its salts through the gills 

 and kidneys, so that there 

 is a net gain of water. But 

 the dogfish and other 

 cartilaginous fishes have 

 adopted the remarkable 

 expedient of retaining 

 much of the urea which 

 results from their nitro- 

 genous metabolism, and so 

 raising the osmotic pressure 

 of their blood till it is 

 slightly above that of sea 

 water. As a result, a 

 moderate amount of water 

 enters their bodies but this 

 is got rid of through the 

 kidneys as in the fresh- 

 water fishes. The retention 

 of urea is brought about 

 b\' a special section of each 

 kidney tubule, which re- 

 absorbs it from the filtrate 

 that is formed by the 

 glomeruli. 



EXCRETION 



The kidnevs lie outside 



the filtrate formed in the glomeruli 

 the abstraction of solids from it 



si. 



or. 



It r. -^j 



ms.ov. 



r.s. 



- 1 m. 



>M 



Fig. 251. — The reproductive organs of a 



female dogfish. The ovary is displaced to 



the animal's left, and only the right 



kidney is shown. 



cl., Cloaca; i.o.rf., internal op>ening of the oviducts ; *., 

 anterior and posterior parts ot right kidney ; ms.n., 

 mesovarium ; o.J., oviduct : oes., o'sophagu- ; or., 

 ovary ; nn., rectum ; 5./., part of the -usfiensor>- 

 ligament of the liver ; s/;., shell gland ; ur. ducts of 

 posterior part of kidney : ii.p.. urinary j apilla ; f.>.. 

 urinary sinus; u:d., Wolffian duct. 



the peritoneum above the 



abdominal cavity, and extend for its whole length (Figs. 251, 252) ; 

 they are wider posteriori}' than in front, and in the female the 

 narrow anterior part is vestigial. The kidney consists of a mass of 



