HABITS OF EARTHWORMS 131 



are associated with it for complete digestion. All of these, like 

 the pharynx, oesophagus, liver, pancreas, and intestine and rec- 

 tal organs, are similarly composed of different tissues, all aiding 

 in the one function of preparing food for the body or of 

 eliminating indigestible and harmful matters. 



Such an aggregate of organs for the performance of a pri- 

 mary function constitutes an organ system. The aggregate of 

 stomach and accompanying organs is termed the digestive 

 or alimentary system. All of the nerve organs together form 

 the nervous system. Similarly we find in all higher animals an 

 excretory system; a supporting and muscular system; a 

 respiratory system; a blood vascular system and a reproduc- 

 tive system, each composed of organs for the performance of 

 one function or part of one function. All of the vital functions, 

 like digestion, secretion, respiration, nervous response, repro- 

 duction, etc., performed by the single cell of Amoeba or Par- 

 amecium, are here performed by complex organ systems. 



All human beings are familiar in a general way with the 

 structures and functions of their own bodies, and they realize, 

 in some degree at least, how complicated and difficult it is to 

 understand human anatomy and human physiology. It is not 

 only desirable but essential, therefore, for the beginner in biol- 

 ogy to become familiar with simple types of organ systems and 

 with some of the factors which have led to the differentiation of 

 such relatively simple into more complex systems, before under- 

 taking a study of the highest animal or deepest biological prob- 

 lem in detail. It is to this end that the present and the follow- 

 ing chapters are devoted. 



II. STRUCTURES AND FUNCTIONS OF THE EARTHWORM, 



LUMBRICUS SP. 



The earthworm occupies a position in the animal scale similar 

 to that occupied by the fern in the plant scale. All the es- 

 sential organ systems are present, but the general organization, 

 while markedly higher than that of Hydra, is relatively simple 

 when compared with Crustacea, insects or vertebrates. 



