CO-ORDINATION. 135 



push the supposed case further, it is evident that 

 if such reflex waves were to occur, the resultincr 

 confusion would very soon require to end in tetanus. 

 As a matter of fact, these reflex waves do not occur ; 

 and the question is, why do they not ? Why is it 

 that a wave is 6nly reinforced in the direction in 

 which it happens to be travelling — so that if, for 

 instance, it happens to start from A in the above 

 series, it is successively propagated by B C in the 

 direction A, B, C, D, and in that direction only ; 

 whereas, if it happens to start from D, it is propa- 

 gated by the same lithocysts in the opposite 

 direction, D, C, B, A, and in that direction only — the 

 wave in the one case terminating at the lithocyst D, 

 and in the other case at the lithocyst A ? Now, 

 although this absence of reflex waves appears at 

 first sight mysterious, it admits of an exceedingly 

 simple explanation. I find that the contractile 

 tissues of the covered-eyed Medusae cannot be made 

 to respond to two successive stimuli of minimal, or 

 but slightly more than minimal intensity, unless 

 such stimuli are separated from one another by 

 a certain considerable interval of time. Now, when 

 in the above illustration the contraction- wave starts 

 from A, by the time it reaches B the portion of 

 tissue included between A and B has just been in 

 contraction in response to the stimulus from A, 

 while the portion of tissue included between B and 

 C has not been in contraction. Consequently, 

 the stimulus resulting from a ganglionic discharge 

 being presumably of minimal, or but slightly more 

 than minimal intensity, the tissue included be- 



