BEHAVIOR OF CCELENTERATA 229 



wave of stimulation to pass to the opposite end. This wave of stimu- 

 lation was followed, if the stimulus was intense, by a wave of contrac- 

 tion; if the stimulus was weak, the wave of stimulation passed alone. 

 This wave caused the tentacles along the margin to contract as it 

 reached them. 



Furthermore, we have seen above that the reaction of the tentacles 

 or of other parts of the body to a given stimulus depends upon the gen- 

 eral physiological state of the body, as determined by the progress of 

 metabolism. Certain tentacles may, through the activity of totally 

 different tentacles, in another region of the body, in supplying material 

 for the metabolic processes, come to react to a given stimulus in a man- 

 ner entirely different from their former reactions. 



The tentacles are therefore not to be compared exclusively to in- 

 dependent organisms associated in a group, but they form parts of a 

 unified organism. While they may react when isolated, they react also 

 under the influence of other parts of the body. We have of course the 

 same condition of affairs in the muscles and various other organs of 

 vertebrates. They may react when isolated, but, like the tentacles 

 of the medusa, they likewise react to influences coming from other parts 

 of the organism. 



The same is true for the manubrium and for other parts of the body. 

 While the isolated manubrium of Gonionemus may react by bending 

 toward food, it shows the same reaction when certain of the tentacles 

 are stimulated by an object moving rapidly across them. The varied 

 reactions of the manubrium to influences affecting other parts of the body 

 are shown most clearly in the experiments of Romanes described on 

 page 201. In Hydra, when the tentacles have seized food, the mouth 

 often begins to open long before the food has reached it. In Metridium, 

 according to Parker, when the tentacles are touched by food, the 

 oesophagus frequently shows peristaltic contractions, and the sphincter of 

 the mouth closes. It is clear that there is a definite coordination and 

 unity in the behavior, brought about by a transmission of stimuli from 

 one part of the body to another. The difference between these organisms 

 and higher animals is in this respect only one of degree. In the ccelen- 

 terates a large share of the behavior is due to the independent reactions 

 of the different organs to the external stimuli, and the transmission of 

 influences from one part of the body to another takes place slowly and 

 without such precision as we find in higher animals. 



The part played by the nervous system in unifying the body we 

 need not take up here, as it has been thoroughly analyzed in the brill- 

 iant work of Romanes (1885), and has been further discussed by Loeb 

 (1900). The essential conclusion to be drawn from the experimental 



