The Urinary System 185 



solved by the development of special collecting apparatuses v^ith ducts 

 leading to the outside. 



Among the invertebrates, there are many specialized excretory 

 organs. No matter how varied their appearance, however, they are all 

 concerned with this fundamental problem of maintaining homeostasis by 

 disposing of waste materials. 



Among the vertebrates, the urinary system is somewhat more 

 complex and there is a special organ, the kidney, composed of numer- 

 ous individual collecting units. This kidney has ducts leading from it 

 to the outside. In many animals, the urine produced by the kidneys 

 accumulates temporarily in a special structure, the bladder. 



_/< 





;b c w^Mmmsm msmm 



Fig. 54. — Diagrams of pro-, meso-, and metanephros. A, Pronephros; 

 B, mesonephros; C, metanephros. 



Three different types of kidneys may be found among the various 

 vertebrate groups. These are known as the pro-, meso-, and meta- 

 nephros. Each fundamentally consists of a series of kidney tubules, the 

 filtration units, in close contact with capillaries. These capillaries form 

 a tuft known as a glomerulus. In the meso- and metanephros, one of 

 these is in contact with each filtration unit. In the pronephros, this fil- 

 tration unit has a ciliated funnel, the nephrostome, which opens into the 

 coelom. The mesonephros is a transitional form: each unit has one 

 branch terminating as a funnel in the coelom and another branch form- 

 ing a capsule in close contact with a glomerulus. This capsule, known 

 as the Bowman's capsule, is the only collecting unit of the metanephros. 



The pronephros is found only in a few primitive fish and in the 

 embryos of most higher forms. It is located at the anterior portion of 

 the coelom, consists of but a few tubules, and is relatively inefficient. 

 The mesonephros is the functional kidney of most fish, amphibians, and 

 the embryos of reptiles, birds, and mammals. It is median in its posi- 



