METABOLISM IN VERTEBRATES 



body into systems, it is convenient to assign a particular function to a single 

 group or system of organs. For that reason, the lungs are discussed as part 

 of the respiratory system, the skin as part of the integumentary system, which 

 functions as a covering for the body, and the liver as part of the digestive 

 system. The excretory organs and their ducts are, however, referred to as 

 the excretory or urinary system. Since the ducts of the excretory organs are 

 also used for the passage of male reproductive cells in vertebrates like the 

 fishes and amphibians, the excretory and reproductive organs are frequently 

 referred to as the urinogenital system. We shall not be concerned here with 

 this dual system but shall describe the structure of the urinary system. 



Two different kinds of functional excretory organs, distinguished by their 

 manner of origin during development, are found in adult vertebrates. The 

 first of these, the mesonephros, is present in adult fishes and amphibians. It 

 appears and disappears during the development of reptiles, birds, and mam- 

 mals, in which a metanephros is formed and functions as the excretory organ 

 of the adult. Although both mesonephroi and metanephroi are often called 

 kidneys, the term should be reserved for metanephroi. The excretory organs 

 are paired structures located behind the peritoneum on the dorsal wall of the 

 coelom and not suspended in it like the stomach and intestine. In mammals 

 a urinary bladder, or reservoir where urine is stored before it is voided, is 

 connected with the kidney by the ureters or excretory ducts (Fig. 3.11). 



Vena cava 



Adrenal 

 gland 



Cortex 



Fig. 3.11. Excretory system and related structures of man. 



57 



