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. 



Aorain, as rei^ards 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 irregularity which is at this stage so frequently 

 observable is, I think, what we should expect to 



