184 



ELEMENTS OF BIOLOGY 



form assumed by nitrogenous materials as they are expelled from 

 the body. It is appropriate at this place to consider the mechanisms 

 whereby the non-gaseous wastes of metabolism are finally expelled 

 from the animal body. In unicellular animals and in the simple 

 Metazoa the wastes are excreted directly by osmosis, for each cell 

 is in direct contact with the water in its environment. But with the 

 development in animals of deeply situated tissues and the appear- 

 ance of hard or tough surface and supporting layers, this direct 



NEPHRID/OPORE 



NEPHROSTOME 



VENTRAL BODY WALL 

 (.INTERNAL VIEW) 



m%\\M\\\\^:, 



SEPTA 



NERVE CORD 



Fig. 126. — The nephridia, excretory organs of the earthworm, diagrammed to show 



position and nature. 



excretion cannot be effected. Therefore associated with this increas- 

 ing complexity is the appearance of organ systems whose special 

 function it is to collect and rid the animal of such wastes. Animal 

 excretory systems occur in a variety of forms; we shall mention 

 only a few for the sake of contrast and to indicate the stages in 

 increasing complexity which precede the system as found in 

 mammals. 



In the earthworm and like animals each segment is equipped with 

 a pair of excretory tubes, termed nephridia (Fig. 126), the inner 

 ends of which are somewhat funnel-shaped and open into the coe- 

 lom. The process of abstracting non-gaseous wastes occurs in the 



