PART II 



BEHAVIOR OF THE LOWER METAZOA 



CHAPTER XI 

 INTRODUCTION AND BEHAVIOR OF CCELENTERATA 



INTRODUCTION 



While unicellular forms are the very lowest organisms, an account 

 limited to their behavior alone might give us a one-sided view of the prin- 

 ciples of behavior in the lower organisms. The Metazoa differ from the 

 Protozoa structurally in the important facts that their bodies are made 

 of many cells and that they have a nervous system. Does the behavior 

 of such organisms differ essentially from that of the Protozoa? Have 

 we been dealing in our study of unicellular organisms with a peculiar 

 group, whose behavior is of a character essentially different from that 

 of other animals ? How far do the general principles to be deduced from 

 the behavior of Protozoa hold for animals in general ? To answer these 

 questions is the province of the following chapters. 



We shall take up in detail the behavior of only one of the lowest 

 groups of Metazoa — the ccelenterates. This will be followed by a 

 chapter on some of the main features of behavior in other invertebrates. 

 A general analysis of behavior in both Protozoa and the lower Metazoa 

 is found in the third part of the book. 



BEHAVIOR OF CCELENTERATA 



The Ccelenterata or Cnidaria form, perhaps, the lowest of the larger 

 groups of Metazoa. This group includes the fresh-water Hydra, 

 hydroids, sea anemones, corals, and jellyfishes or medusae. The 

 behavior of the corals and of hydroids has been comparatively little 

 studied, so that the present account will be limited mainly to Hydra, the 

 sea anemones, and medusae. 



All of these animals are made up of many cells, of many different 

 kinds, and usually arranged in three more or less irregular layers. Of 



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