64 
SCIENCE. 
PHYSIOLOGY OF THE FRESH WATER MEDUSA. 
The structure of this remarkable animal has already been 
investigated and described by Professors Allman and Lan- 
kester, with the result of showing that, although constitut- 
ing a new genus, it is in all respects a true Medusa. After 
the publication of their papers 1 began to work out the 
physiology of the new form, and the following arc the 
results which so far I have obtained. 
The natural movements of the Medusa precisely resem- 
ble those of its marine congeners. More particularly, these 
movements resemble those of the marine species which do 
not swim continuously, but indulge in frequent pauses. In 
water at the temperature of that in the Victoria Lily-house 
(85° F.) the pauses are frequent, and the rate of the rhytnm 
irregular — suddenly quickening and suddenly slowing even 
during the same bout, which has the effect of giving an 
almost intelligent appearance to the movements. This is 
especially the case with young specimens. In colder water 
(65° to 75 0 ) the movements are more regular and sustained ; 
so that, guided by the analogy furnished by my experiments 
on the marine forms, I infer that the temperature of the 
natural habitat of this Medusa cannot be so high as that of 
the water in the Victoria Lily-house. In water at that tem- 
perature the rate of the rhythm is enormously high, some- 
times rising to three pulsations per second. But by pro- 
gressively cooling the water, this rate may be progressively 
lowered, just as in the case of the marine species ; and in 
water at 65° the maximum rate that I have observed is 
eighty pulsations per minute. As the temperature at which 
the greatest activity is displayed by the freshwater species 
is a temperature so high as to be fatal to all the marine 
species which I have observed, the effects of cooling are of 
course only parallel in the two cases when the effects of a 
series of high temperatures in the oue case are compared 
with those of a series of lower temperatures in the other. 
Similarly, while a temperature of 70° is fatal to all the 
species of marine Medusae which I have examined, it is 
only a temperature of 100 degrees that is fatal to the fresh- 
water species. Lastly, while the marine species will endure 
any degree of cold without loss of life, such is not the case 
with the freshwater species. Marine Medusae, after having 
been frozen solid, will, when gradually thawed out, again 
resume their swimming movements; but this freshwater 
Medusa is completely destroyed by freezing. Upon being 
thawed out, the animal is seen to have shrunk into a tiny 
ball, and it never again recovers either its life or its shape. 
The animal seeks the sunlight. If one end of the tank is 
shaded, all the Medusae congregate at the end which re- 
mains unshaded. Moreover, during the daytime they swim 
about at the surface of the water; but when the sun goes 
down they subside, and can no longer be seen. In all 
these habits they resemble many of the sea-water species. 
They are themselves non-luminous. 
I have tried on about a dozen specimens the effect of 
excising the margin of the nectocalyx. In the case of all 
the specimens ttius operated upon, the result was the same, 
and corresponded precisely with that which I have obtained 
in the case of marine species. That is to say, the operation 
produces immediate, total, and permanent paralysis of the 
nectocalyx, while the severed margin continues to pulsate 
for two or three days. The excitability of a nectocalyx 
thus mutilated persists for a day or two, and then gradually 
dies out — thus also resembling the case of the marine 
naked-eyed Medusae. Mare particu arlv, this excitability 
resembles that of those marine species which sometimes 
respond to a single stimulation witn two or three successive 
contractions. 
A point of specially physiological interest may be here 
noticed. In its unmutilated state the freshwater Medusa 
exhibits the power of localizing with its manubrium a seat 
of stimulation situated in the bell. That is to say, when a 
part of the bell is nipped with the forceps, or otherwise 
irritated, the free end of the manubrium is moved over and 
applied to the part irritated. So far, the movement of 
localization is precisely similar to that which I have pre- 
viously described as occurring in Tiaropsis indicans (Phi/. 
Trans., vol. clxvii.) But further than this, I find a curious 
difference. For while in T. indicans these movements of 
localization continue unimpaired after the margin of the 
bell has been removed, and will be ineffectually attempted 
even after the bell is almost entirely cut away from its con- 
nections with the manubrium ; in the freshwater Medusa 
these movements of localization cease after the extreme 
margin of the bell has been removed. For some reason or 
another the integrity of the margin here seems to be neces- 
sary for exciting the manubrium to perform its movements 
of localization. It is clear that this reason must either be 
that the margin contains the nerve-centres which preside 
over these localizing movements of the manubrium, or 
much more probably, that it contains some peripheral 
nervous structures which are alone capable of transmitting 
to the manubrium a stimulus adequate to evoke the move- 
ments of localization. In its unmutilated state this Medusa 
is at intervals perpetually applying the extremity of its 
manubrium to one part or another of the margin of the bell, 
the part of the margin touched always bending in to meet 
the approaching extremity of the manubrium. In some 
cases it can be seen that the object of this co-ordinated 
movement is to allow the extremity of the manubrium — i. e., 
the mouth of the animal — to pick off a small particle of 
food that has become entangled in the marginal tentacles. 
It is therefore not improbable that in all cases this is the 
object of such movements, although in most cases the 
particle which is caught by the tentacles is too small to be 
seen with the naked eye. As it is thus no doubt a matter 
of great importance in the economy of this Medusa that its 
marginal tentacles should be very sensitive to contact with 
minute particles, so that a very slight stimulus applied to 
them should start the co-ordinated movements of localiza- 
tion, it is not surprising that the tentacular rim should 
present nerve-endings so far sensitive that only by their 
excitation can the reflex mechanism be thrown into action. 
But if such is the explanation in this case, it is curious that 
in Tiaropsis indicans every part of the bell should be equally 
capable of yielding a stimulus to a precisely similar reflex 
action. 
In pursuance of this point I tried the experiment of cut- 
ting off portions of the margin, and stimulating the bell 
above the portions of the margin which 1 had removed. I found 
that in this case the manubrium did not remain passive as 
it did when the whole margin of the bell was removed ; 
but that it made ineffectual efforts to find the offending 
body, and in doing so always touched some part of the 
margin which was still unmutilated. I can only explain 
this fact by supposing that the stimulus supplied to the 
mutilated part is spread over the bell, and falsely referred 
by the manubrium to some part of the sensitive— i.e., un- 
mutilated — margin. 
But to complete this account of the localising movements 
it is necessary to state one additional fact which, for the 
sake of clearness, I have hitherto omitted. If any one of 
the four radial tubes is irritated, the manubrium will cor- 
rectly localise the seat of irritation, whether or not the margin 
of the bell has been previously removed. This greater case, 
so to speak, of localising stimuli in the course of the radial 
tubes rather than anywhere else in the umbrella except the 
margin, corresponds with what I found to be the case in 
T. indicans, and probably has a direct reference to the dis- 
tribution of the principal nerve-tracis. 
On the whole, therefore, contrasting this case of localisa- 
tion with the closely parallel case presented by T. indicans, 
I should say that the two chiefly differ in the freshwater 
Medusa, even when unmutilated, not being able to localise 
so promptly or so certainly : and in the localisation being 
only performed with reference to the margin and radial 
tubes, instead of with reference to the whole excitable sur- 
face of the animal. 
All marine Medusae are very intolerant of freshwater, and 
therefore as the fresh waier species most presumably have 
had mariae ancestors, 1 it seemed an interesting question to 
determine how far this species would prove tolerant of sea 
water. For the sake of comparison I shall first briefly de- 
scribe the effects of fresh water upon the marine species. 2 
If a naked eyed Medusa which is swimming actively in sea 
water is suddenly transferred to fresh water, it will instant- 
aneously collapse, become motionless, and sink to the 
1 Looking to the enormous number of marine species of Medusae, it is 
much more proba le that the freshwater species were derived from them, 
than that they were derived from freshwater ancestry. 
2 For full account, see Phil. Trans., vol. clxvii., pp. 744-745, 
