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ME. G. J. EOMANES ON THE LOCOMOTOE SYSTEM OF MEDUSiE. 
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 find if this view as 
to the essentially reflex character of the natural rhythm is the true one. 
If this view is the true one, the question next arises as to the nature of the process 
which goes on in the excitable tissues, and which afterwards acts as a stimulus on the 
ganglionic tissues. This question, however, I am quite unable to answer. Whether 
the process is one of nutrition, of oxygenation, of chemical changes exerted by the sea- 
water, or a process of any other kind, further experiments may perhaps be able to 
show ; but meanwhile, I have no suggestion to offer. 
§ 5. Effects of lessening the amount of Tissue adhering to a single Ganglion *. — 
The above experiments led me to try the effects of cutting out a single lithocyst of 
Aurelia , and, after the rhythm of the detached segment had become regular, progres- 
sively paring down the contractile tissues around the ganglion. I found that this pro- 
cess had no very marked effect on the rhythm until the paring reached within an inch 
or two of the ganglion. Then, however, the effect began to show itself, and with every 
successive paring it became more marked. This effect consisted in slowing the rate of 
the rhythm, but more especially in giving rise to prolonged pauses. Indeed, if only a 
very little contractile tissue was left adhering to the ganglion, the pauses often became 
immensely prolonged ; so that one might almost suppose the ganglion to have entirely 
ceased discharging. But if a stimulus of any kind were then applied, the rhythmic dis- 
charges at once recommenced. They then generally continued for some little time at a 
slower rate than that which they had manifested before they were affected by the paring 
down of the contractile tissue. To illustrate these effects, it will suffice to quote one experi- 
ment from my notes “ In an eighth-part segment of Aurelia, measuring 5 inches by 4, 
the rhythm for several hours was quite regular at 18 per minute. A band of tissue measur- 
ing half an inch across was now removed from each side of the segment. Bate immediately 
rose to 22, and then gradually fell again to 18. Another similar band was now removed. 
This operation was followed by a pause of several minutes’ duration, after which some 
irregular contractions occurred ; but eventually these gave place to a regular rhythm of 
16 per minute. The tissue-cone was now truncated by cutting off an inch of its apex. 
The rate of rhythm rose to 18, and eventually became regular at 13. On now again 
truncating the cone, by removing from its upper part a band of tissue half an inch wide, 
the rate again rose slightly, but then became stationary at 10. Lastly, a band of tissue 
half an inch wide was removed from both the sides of the truncated cone, thus reducing 
the contractile tissue surrounding the lithocyst to a small piece measuring -f of an inch 
long by \ an inch broad. This final operation was followed by prolonged periods of 
* In view of more recent observations on the histology of lithocysts, I substitute in this paper the term 
“ ganglion ” for the term “locomotor centre,” which in my previous paper I everywhere used to denote these 
bodies. 
