104 RESPIRATION AND DISPOSAL OF WASTES 



organ. In other unicellular animals, and even in the 

 hydra-like animals and the sponges, the excretions are 

 removed from the body without the aid of any special 

 organs. Elsewhere in the animal kingdom there usually 

 are definite excretory organs called kidneys, that in many 

 special cases cooperate with organs having other functions 

 as well. The digestive tract of worms, the liver of num- 

 erous snails, clams, and vertebrates, the walls of the heart 

 and neighboring blood vessels all play a minor though 

 often a very important part in the excretory process. 



In a few groups of animals a portion of the waste 

 products are not removed from the body, but in the form 

 of variously colored pigments are deposited in the skin. 

 The wings of many butterflies owe their color to such 

 compounds, and the silvery sheen of a fish's body is like- 

 wise due to an otherwise useless substance embedded in 

 the scales. 



Excretory Organs of Lower Animals. — The differ- 

 ent types of excretory organs among the lower animals 

 are many, and their modes of operation are equally di- 

 verse. In the flatworms they take the form of many 

 widely scattered cells which in form appear much like 

 highly branched amoebae. The branches extend into the 

 spaces between the tissue cells, collect their wastes into 

 a droplet that is periodically discharged into a delicate 

 tube with which each cell is in contact. A flagellum at 

 the junction of cell and tube drives the wastes along into 

 larger vessels that finally open through the body- wall 

 by one or more pores. 



In the segmented worms, such as the earthworm, in the 

 snails, clams, and their allies, the kidneys (one or more 

 in number) present the form of more or less voluminous 

 tubes, one end of which communicates with the exterior 

 while the opposite extremity opens into the body cavity. 

 The main portion of this organ is either directly bathed 

 with blood or is supplied with capillaries from which the 

 wastes are extracted by the kidney cells and are thereupon 



