400 GENERAL ZOOLOGY. 



muscular partition (diaphragm) which divides the body- 

 cavity becomes an efficient organ in the process (see p. 309). 



We naturally think of work in terms of motion, and in 

 the case of an animal the contraction of a muscle or the 

 movement of a part or the whole of the body naturally 

 suggest themselves as examples. These, however, are 

 but a part of the work which the animal does. The per- 

 formance of any function of the body is really work. 

 When a gland secretes, a nerve acts, an intestine absorbs, 

 or the mind carries on its operations, the expenditure of 

 energy is called for just as in the contraction of a muscle. 

 So all parts must have both food and oxygen. 



When coal is burned in an engine, besides energy there 

 is a production of waste. A part of this waste passes off 

 in a gaseous condition as water vapor and part as ashes. 

 When any part of the animal body works there is a similar 

 formation of waste, and the carbon dioxide and water 

 vapor are carried away by the same structures (tracheae 

 in the insects, blood-vessels and gills or lungs in many 

 other forms) which brought the oxygen to the parts. 



The animal, unlike the engine, needs also for its fuel 

 substances known to the chemist as nitrogenous food 

 (proteids) and the combustion of this produces, besides 

 the carbon dioxide and water, nitrogenous waste, and 

 this, in all of the higher animals, is eliminated by means 

 of organs which can be grouped under the common name 

 of excretory organs. Here are to be placed not only 

 those structures specifically called kidneys in the fore- 

 going pages, but also the green gland of the crayfish, the 

 Malpighian tubes of insects, the nephridia of the earth- 

 worm, and the organ of Bo j anus in the clam. Even the 

 contractile vacuole of the Protozoa seems to be an organ 

 for the excretion of nitrogenous waste. 



