CHAPTER 



17 



THE VITAL FUNCTIONS 



AND ORGAN SYSTEMS 



OF INVERTEBRATES 



The vital functions and organ systems of vertebrate animals have been dis- 

 cussed in the early chapters of this book, and in subsequent chapters com- 

 parisons have been drawn between the organ systems of vertebrates and the 

 structures serving similar functions in invertebrates. Let us now consider 

 the functional systems of invertebrates in more general terms; such a survey 

 will illustrate the kind of diversity and unity that is everywhere apparent 

 when the world of animal life is carefully studied. Often the plan of an 

 organ system is remarkably similar throughout a considerable number of 

 phyla. But in other phyla the varied structural relationships clearly indicate 

 that the systems in these phyla are not homologous, although non-homologous 

 parts may be functionally analogous. It should be recalled that the simpler 

 and more primitive types of invertebrate animals do not possess organ systems, 

 if we define these as groups of organs which, taken as a whole, perform some 

 common function. Indeed, the simpler animals may even lack organs, and 

 the simplest types of metazoans have not evolved tissues. Nevertheless, one 

 important generalization emerges from comparative studies of invertebrates, 

 and this will be the theme of the present chapter. All animals, of whatever 

 grade of complexity, are (figuratively speaking) faced with certain basic 

 problems of existence; and different groups of animals have solved these 

 common problems in a variety of ways, utilizing the structural possibilities 

 available to them within their particular levels of organization. For example, 

 the metabolism of all animals imposes certain general requirements: all 

 animals must have sources of energy to maintain their metabolic reactions. 

 In simple, plant-like protozoans, energy-rich compounds are manufactured 

 by a photosynthetic process; in more typical animals, metabolic requirements 

 are met by the many activities involved in the capture, ingestion, digestion, 

 absorption, circulation, and assimilation of energy-rich compounds which 



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