164° JELLY-FISH, STAR-FISH, AND SEA-URCHINS, 
effect of such an operation as we are considering is 
greatly to accelerate the rhythm, and that this 
acceleration then gradually declines through a period 
of about a quarter of an hour. These facts tend to 
support this view, because, if it is correct, they are 
what we might anticipate. If the manubrium, for 
instance, while in situ is continually supplying a 
gentle stimulus to the marginal ganglia, when it is 
suddenly cut off, the nerve-tracts through which 
this stimulating influence had previously been con- 
veyed must be cut through; and as it is well known 
how irritable nerve-fibres are at their points of 
section, it is to be expected that the irritation caused 
by cutting these nerve-tracts, and probably also by 
the action of the sea-water on their cut extremities, 
would cause them to stimulate the ganglia more 
powerfully than they did before their mutilation. 
And here I may state that on several occasions, 
with vigorous specimens, I have observed a sudden 
removal of the manubrium to be followed, not 
merely with a quickening of the rhythm on the 
part of the bell, but with a violent and long-sustained 
spasm. 
Again, as regards the other fact before us, it is 
obvious that as soon as the cut extremities of the 
nerves begin to die down, and so gradually to lose 
their irritability, the effect on the rhythm would be 
just what we observe it to be, viz.a gradual slowing 
till the rate falls considerably below that which was 
exhibited by the unmutilated animal. And even 
the wrregularity which is at this stage so frequently 
observable is, I think, what we should expect to 
