382 sEusr steuciube and excretion 



the common external opening for the kidney and the intes- 

 tine. In mammals the openings are for the most part dis- 

 tinct, the kidney opening being called the urinary opening. 

 The development of the bladder is simply a device for pre- 

 venting continuous secretion and permitting the animal 

 to control this operation of removal from the body. 



In general, then, whether we are dealing with the 

 nephridium of the worm or the sweat gland of man, the 

 means of disposal of waste are practically identical in action, 

 and the kidneys are to be considered as simply a collection 

 of separate excretory glands enormously increased in number 

 to provide for the increase in waste, and having a common 

 duct for removal. 



