ORGANIC MOVEMENTS 31 



nervous function, besides mere conduction, seems to be at 

 work. 



But, unfortunately again, the simple scheme fails us, as 

 soon as the limits of mere typical progressive motions are 

 transgressed. The sea-urchin, for instance, very properly 

 follows our law when simply walking, but something very 

 different happens as soon as it is put on its back and has 

 to turn over into its normal position ; all sorts of new 

 elemental functions, relating to the dependence of the 

 different single motor constituents on one another, are 

 playing their part here, just as circumstances require, and 

 the stating of a simple formula becomes an impossibility. 

 The same holds for the turning over of the starfish, in 

 which the successful movements of some of the arms stop 

 the movement of the others, and, indeed, we properly can 

 say, that almost any movement of an animal, in any way 

 deserving the name " abnormal," shows a particular type of 

 motor combination. 



The " righting reactions ' of the starfish and certain 

 other points of interest form the subject of a recent very 

 important memoir by Jennings (Univ. Calif orn. Publ. Zool. 

 4, 1907, p. 53). Jennings fully confirms the older results 

 published by Preyer, and adds a good number of new 

 results. Let me mention only a few topics. Asterias was 

 found to avoid obstacles whilst creeping to a certain place 

 in a known environment, but to " explore " every object 

 in new surroundings. The " righting reaction ' may show 

 a great many very different types. In each case the initial 

 movement of each single arm is determined separately by 

 external stimuli or internal conditions, but as soon as the least 

 result with regard to righting is reached a " unified impulse * 



