XXIV EXCRETORY ORGANS 281 



its oxygen to the tissues. On the other hand, whenever the 

 blood is brought sufficiently near the external air — or water 

 in the case of an aquatic animal — the opposite process takes 

 place, oxygen being absorbed and carbon dioxide given off. 

 Hemoglobin is therefore to be looked upon as a respiratory 

 or oxygen-carrying pigment ; its function is to provide the 

 various parts of the body with a constant supply of oxygen, 

 while the carbon dioxide formed by their oxidation is given 

 up to the blood. The particular part of the body in which 

 the carbon dioxide accumulated in the blood is exchanged 

 for the oxygen of the surrounding medium is called a 

 respiratory organ ; in Polygordius, as in the earthworm and 

 many other of the lower animals, there is no specialised 

 respiratory organ — lung or gill — but the necessary exchange 

 of gases is performed by the entire surface of the body. 



In discussing in a previous lesson the differences between 

 plants and animals, we found (p. 178) that in the unicellular 

 organisms previously studied, the presence of an excretory 

 organ in the form of a contractile vacuole was a characteristic 

 feature of such undoubted animals as the ciliate Infusoria, 

 but was absent in such undoubted plants as Vaucheriaand 

 Mucor. But the reader will have noticed that Hydra and its 

 allies have no specialised excretory organ, waste products 

 being apparently discharged from any part of the surface. 

 In Polygordius we meet once more with an animal in which 

 excretory organs are present, although, in correspondence 

 with the complexity of the animal itself, they are very 

 different from the simple contractile vacuoles of Paramoe- 

 cium or Vorticella. 



The excretory organs of Polygordius consist of little tubes 

 called nephridia, of which each metamere possesses a pair, 

 one on either side (Fig. 67, a, b, and c, Nphm). Each 



