INTRODUCTION TO THE METAZOA 69 



aided in carrying out its functions by the other tissues of the 

 same organ. 



Thus, in a digestive organ the essential process of digestion is 

 made possible through the presence of glandular epithelia which 

 form substances necessary for the digestion of food. The 

 epithelia are frequently aided in this function by cooperative 

 muscles under whose action the content of the digestive tract is 

 kept agitated so that all of the food material comes into contact 

 with the digestive fluids produced by the epithelia. 



When identical or similar organs are grouped for the perform- 

 ance of the same or related functions, such a complex is termed 

 an organ system. All of the activities which characterize the 

 living animal are so fundamentally interrelated that the numer- 

 ous organs and tissues of the metazoan body are capable of being 

 grouped under a relatively small number of systems. Yet these 

 major organ groups, the systems, have their interrelations and 

 function not as independent units but as a correlated whole — 

 the organism or the individual. 



Throughout the following sections of this book constant 

 reference is made to the digestive, circulatory, respiratory, 

 excretory, locomotor, reproductive, and nervous systems. The 

 organs of which each system is composed in the various phyla and 

 classes are not necessarily identical in structure or origin, for 

 frequently the same function is carried on by conspicuously 

 different organs in different groups. A brief discussion of the 

 modifications of the several organ systems is given here, but for a 

 full understanding and appreciation of this discussion, the 

 student should again read this section after having completed the 

 laboratory study of a number of metazoan forms. 



The Digestive System. — The power of transforming non-living 

 food substances into living matter or protoplasm is a distinctive 

 property of all living organisms. This is not accomplished 

 by a direct transmutation or by a simple act of endowing the 

 food with life. Since living protoplasm is in a liquid or semi- 

 liquid state the process of digestion or liquefying of food is 

 the essential first step in the transformation of food into living 

 matter. In some degenerate forms of animal life which have 

 become adapted to a purely parasitic existence, as, for example, 

 the tapeworms and the Acanthocephala, food taking is restricted 

 to the direct absorption of liquids by the cells of the body. 

 Organs of digestion are entirely wanting in such instances, 



