DIGESTIVE ORGANS. 53 



nutrient fluid (blood) which permeates the body, and is carried in 

 more or less definite tracts to all the organs. To the latter the 

 blood yields its ingredients, and receives from them such decom- 

 position products as have become useless, and carries them away to 

 be excreted in definite organs. The organs which serve for the 

 performance of the different functions of nutrition and excretion 



FIG. 41. Rotalia veneta (after M. Schultze) wibh a diatora caught in the pseurtopodial 



network. 



consist of the apparatus for the reception of food and for its diges- 

 tion, and for blood formation ; and of the organs of circulation, 

 respiration, and of excretion. 



Digestive organs. Even animals which have only the value of a 

 single cell (Protozoa) swallow solid particles of food. This is effected 



