CO-ORDINATION. 139 
lionie 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 Meduse, 
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 very liable to be 
thrown out of gear, so that when the animals are 
suffering in the least degree from any injurious 
conditions, often too slight 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 manubrium does not interfere with this steering 
action ; but 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 in situ. 
