196 
MR. a. J. ROMANES ON THE LOCOMOTOR SYSTEM OF MEDUSAE. 
of nerve-fibres or only upon that of muscle-fibres. But owing to the impossibility of 
procuring a stained preparation of such a regenerated injury, I cannot speak with 
confidence upon this point. I can only say that the nerves which admit of being seen 
without staining never pass across the line of incision till long after the contractile 
waves are able to do so. But, of course, this does not prove that the more delicate 
nerves which are only brought into view by staining may not have previously passed 
across the line of incision, and have thus been instrumental in the re-establishment of 
physiological continuity. 
I may here observe that, so far as my experiments in this connexion have gone, it 
appears to be an essential condition to the regeneration of the excitable tissues that 
the whole thickness of the gelatinous tissues should not have been severed. That is 
to say, I have never succeeded, though I have made a number of trials, in obtaining 
union between any two parts of a Medusa which have been completely severed from 
one another and are then held in close apposition by stitching. So far as I have seen, 
it is only when the gelatinous tissues of the umbrella are left intact, or, at least, are 
not severely injured, that the injuries in the excitable tissues will admit of being 
repaired. 
III. GENERAL SUMMARY. 
Artificial rhythm may be produced in various species both of covered- and of naked¬ 
eyed Medusae, though in some species electrical and in other species chemical stimula¬ 
tion is most effective for this purpose. In all cases, however, the stimulation which is 
supplied must be constant and of not more, or but slightly more, than minimal inten¬ 
sity. In Aurelia artificial rhythm is best produced by employing weak faradaic 
stimulation, when the resulting rhythm is often of a most regular and persistent 
character. Eventually, however, exhaustion produces irregularity and cessation of the 
rhythm, and prolonged rest is then required before any perfect rhythm can be again 
produced by the faradaic stimulation. Increasing the strength of the current within 
certain limits increases the rate of the rhythm. In Sarsia artificial rhythm may be 
best produced by employing weak chemical stimulation. Soon after the concave sur¬ 
face of the paralyzed bell becomes opalescent, the shivering movements begin, and 
continue without intermission for a variable time. Shortly after transference of the 
bell to normal sea-water these movements cease, to be again resumed if the bell is 
again transferred to acidulated water. The paralyzed bell of Sarsia will sometimes 
manifest these rhythmic movements even in normal sea-water, provided that it has 
first been injured by the application of either the constant or the induced current. 
The polypite of Sarsia will also manifest a siow and long-continued rhythm as a 
result of mechanical or chemical injury. The umbrella of Aurelia will sometimes give 
three or four rhythmic contractions in response to a single mechanical stimulation, and 
will sometimes also respond rhythmically to the constant current. Thus all modes of 
constant stimulation (except heat) may produce artificial rhythm in one or other of 
these two species of Medusm. 
