128 ORGANS AND ORGAN SYSTEMS 



the pharynx, oesophagus, liver, pancreas, and intestine and rectal 

 organs are similarly composed of different tissues, all aiding in 

 the one function of preparing food for the body or eliminating 

 indigestible and harmful matters. 



Such an aggregate of organs for the performance of a primary 

 function constitutes an organ system. The aggregate of 

 stomach and accompanying organs is termed the digestive 

 system or alimentary system. All of the nerve organs together 

 form the nervous system. Similarly we find in all higher an - 

 mals an excretory system; a supporting and muscular system; 

 a respiratory system; a blood vascular system and a repro- 

 ductive system, all composed of organs for the performance of 

 one function or part of one function each. All of the vital 

 functions like digestion, secretion, respiration, nervous response, 

 reproduction, etc., performed by the single cell of amoeba or 

 paramecium 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 biology 

 to become familiar with simple types of organ systems and 

 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 

 problem in detail. It is to this end that the present and the 

 following 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. 



A. OCCURRENCE, HABITS AND MODE OF LIFE OF EARTH- 

 WORMS. Earthworms are widely distributed over the earth and 



