45^ H. S. 'Jennings. 



hungry specimens. The well fed animal reacts much less readily 

 and strongly to simple mechanical shock. If touched with a 

 needle the well fed individual of Aiptasia either does not react at 

 all, or contracts very slightly, while the hungry specimen reacts 

 suddenly and powerfully. A slight disturbance in the water has 

 no effect on the well fed individual, while the hungry one contracts 

 strongly. To chemical stimuli the same relations apply. A much 

 stronger solution of any given chemical is required in order to 

 produce contraction in the well fed individual, as compared with 

 the hungry one. The bearing of such facts on quantitative 

 determinations in reaction work is evident. If we should attempt 

 to determine the strength of a given chemical which causes con- 

 traction in Aiptasia, we should obtain totally different results, 

 according as we used specimens that were very hungry, moderately 

 hungry, or thoroughly satiated. No "normal" concentration 

 for causing reaction could be determined for even a single given 

 specimen, for the state of metabolism, and with it the tendency 

 to react, is continually changing. 



It is, of course, clear that the change due to varying metabolic 

 states cannot be interpreted alone as a general increase or decrease 

 of sensitiveness. Much more significant is the complete qualita- 

 tive change in the nature of the reaction to a certain stimulus, 

 due to this cause, which we have seen both in Stoichactis and in 

 Aiptasia. 



2. ACCLIMATIZATION TO STIMULI. 



Sea anemones show acclimatization to stimuli in the same way 

 as do the protozoan Stentor and many other low organisms. 

 A light stimulus that is not injurious may cause at first a strong 

 reaction, then on repetition produce no reaction at all, or a very 

 slight one. This is easily shown with Aiptasia annulata in the 

 following way: A specimen is selected with outspread disk close 

 beneath the surface of the water. From a height of about 30 cm. 

 a drop of water is allowed to fall on the water surface just above 

 the disk. At once the animal contracts strongly. Waiting till 

 it has expanded again, another drop is allowed to fall in the same 

 way. As a rule there is no reaction to this or to succeeding drops. 

 Sometimes there is a response to the first two or even three drops, 

 but usually there is no reaction after the first one. A slight 



