ME. Gr. J. EOMANES ON THE LOCOMOTOE SYSTEM OE MEDUSAE. 
781 
Medusae, the slightest want of ganglionic coordination exhibited by any of the specimens. 
Moreover, that the ganglionic coordination is in this case wonderfully far advanced, is 
proved by the fact of members of this genus being able to steer themselves while 
following a light, as described in my last paper*. 
In the discophorous species of naked-eyed Medusae, however, perfectly coordinated 
action is by no means of such invariable occurrence as it is in Sarsia ; for although in 
perfectly healthy and vigorous specimens systole and diastole occur at the same instant 
over the whole nectocalyx, this harmoniously acting mechanism is very liable to be 
thrown out of gear ; so that when the animals are suffering in the least degree from 
any injurious conditions — often too slight and obscure to admit of discernment — the 
swimming movements are no longer synchronous over the whole nectocalyx ; but now 
one part is in systole while another part is in diastole, and now several parts may be in 
diastole while other parts are in systole. And, as in these animals very slight causes 
seem sufficient thus to impair the ganglionic coordination, it generally happens that in 
a bell-jar containing a number of specimens belonging to different species, numerous 
examples of more or less irregular swimming movements are observable f. 
(b) Taking, then, the case of Sarsia first, from my previous observations on the phy- 
siological harmony subsisting between the tentacles, I was led to expect that the coor- 
dination of the locomotor ganglia was probably effected by means of the same tissue- 
tracts through which the intertentacular harmony was effected, viz. those situated in 
the margin of the bell. Accordingly, I introduced four short radial cuts, one midway 
between each pair of adjacent marginal bodies. The coordination, however, was not 
* Eemoving the polypite does not interfere with this steering action ; but if any considerable portion of the 
margin is excised, the animal seems no longer able to find the beam of light, even though one or more of the 
marginal bodies are left in situ. 
t I think it is worth while to describe a peculiar irregularity of a very definite kind which is occasionally 
observable in Tiaropsis diademata. I have only noticed it in the case of tolerably vigorous specimens. It 
consists in the simultaneous contraction of two opposite quadrants of the nectocalyx, while the other two 
quadrants remain passive. The effect of the systole is therefore to convert the whole nectocalyx into an almost 
linear form. Diastole then follows as usual ; but in the next systole the two quadrants which had previously 
been passive now contract, while the two which had previously contracted now remain passive. The necto- 
calyx, therefore, again assumes a linear form, but this time in a direction at right angles to that in which it 
had previously done so. Diastole then again ensues, and the third systole resembles the first one, the fourth 
the second, and so on — the nectocalyx alternately assuming the linear form in each of the two opposite direc- 
tions. These peculiar motions may continue for a long time without interruption. Their rhythm is always 
perfect, and in its rate precisely resembles that which the same animal exhibits when swimming normally. 
The comparison is easily made, because every now and then the abnormal motions become suddenly converted 
into the normal ones, and vice versa. The time during which a normal or an abnormal bout continues is 
variable. The manifestation of such abnormal swimming-bouts appears to be an idiosyncracy confined to a 
very small percentage of individuals; for, while one may observe a thousand specimens for any length of time 
without perceiving the occurrence of these peculiar movements, if there happens to be an additional specimen 
presenting the idiosyncracy in question, every time the observer goes to look at it he may feel that there is 
about an equal chance of his seeing the normal or the abnormal movements. 
5 K 
MDCCCLXXVII. 
