ORGANIC MOVEMENTS 29 



TJie Classes of Co-ordination 



And now let us glance at the different types or classes 

 of co-ordinated animal motions, always asking at each step, 

 what would be intelligible here on the theory of a machine 

 and what would not. 



The simplest class considered logically of all co- 

 ordinated movements is formed by the so-called " chain- 

 reflexes," which seem to occur in several groups of Inverte- 

 brates ; one typical simple reflex is combined here with a 

 number of others in a fixed way. Either as in Medusae 

 or in the heart of higher animals one simple reflex causes 

 the simultaneous performance of many similar equal ones, 

 or the end of the performance of one is the stimulus to 

 the performance of the other, as in the movements of 

 many so-called metameric animals. We may speak of 

 " synchronic " reflexes in the first case and " metachronic ' 

 ones in the second. In the jelly-fish all the parts of the 

 " umbrella " move together as soon as one of them has 

 begun movement, and in the earthworm the end of the 

 contraction of one segment always causes the next one to 

 move. And it may happen that parts of an animal which 

 are dissimilar in organisation may also appear as the single 

 constituents of a metachronic chain-reflex. It is especially 

 to J. Loeb * that much of our knowledge of " chain-reflexes " 



both Uexkuell and Jennings are constantly at literary warfare with one 

 another. But it seems to me that this is owing to a mutual misunderstanding, 

 In any case von Uexkuell does not operate with the old concept of " reflex " 

 exclusively ; his important discrimination between two elemental functions 

 of muscles and motor nerves ordinary contraction and "Sperrung" would 

 by itself suffice to show that. 



1 Comparative Physiology of the Brain and Comparative Psychology, New 

 York, 1900. 



