CO-OKDINATION. 139 



lionic co-ordination exhibited by any of the speci- 

 mens. Moreover, that the ganglionic co-ordination 

 is in this case wonderfully far advanced is proved 

 by the fact of members of this genus being able 

 to steer themselves while following a light, as 

 previously described.* 



In the discophorous species of naked-eyed Medusae, 

 however, perfectly co-ordinated action is by no 

 means of such invariable occurrence as it is in 

 Sarsia; for although in perfectly healthy and 

 vigorous specimens systole and diastole occur at 

 the same instant over the whole nectocalyx, this 

 harmoniously acting mechanism is ver}^ liable to be 

 thrown out of gear, so that when the animals are 

 suffering in the least degree from any injurious 

 conditions, often too slio'ht and obscure to admit of 

 discernment, the swimming movements are no longer 

 synchronous over the whole nectocalyx; but now 

 one part is in systole while another part is in 

 diastole, and now several parts may be in diastole 

 while other parts are in systole. And as in these 

 animals very slight causes seem sufficient thus to 

 impair the ganglionic co-ordination, it generally 

 happens that in a bell-jar containing a number of 

 specimens belonging to different species, numerous 

 examples of more or less irregular swimming move- 

 ments are observable. 



Taking, then, the case of Sarsia first, from my 



* Removing the mannbrinm does not interfere with this steerin^j 

 action ; bat if any considerable portion of the margin is excised, 

 the animal seems no longer able to find the beam of light, even 

 though one or more of the marginal bodies be left iyi situ. 



