In healthy medusae chloroform first arrests the spontaneous movements of the bell. When now irritated it answers by a single contraction, instead of by a series, to such stimulation.

After the bell has ceased to respond, nipping its margin causes the polyp to contract.

After stimulation of any part of the bell ceases to produce response in any part of the organism, the polyp will respond to stimuli directly applied to it. Nitrite of amyl also produces effects which in many respects are similar to those of chloroform. There are, however, certain exceptions; the first is that, before the spontaneous movements are abolished, the rhythm becomes quickened, and the strength of the pulsations is diminished. The movements also die out more gradually than under chloroform, and before they entirely cease they become localised to the muscular tissue close to the margin. When the dose is large, spasmodic contractions are produced which obliterate the gradual paralysing action of the drug.

Caffeine first causes an increase in the rate of pulsation, and diminishes its strength after a few seconds. This condition passes off, and the spontaneous movements become gradually abolished. They still remain for a long time sensitive to stimulation, and at first respond by several feeble contractions to each stimulus; afterwards by a single response; and afterwards they do not respond at all.

As medusae paralysed by removal of the ganglia never respond to a single stimulus with more than a single contraction, the increased number of contractions which at first appear after the application of the stimulus, are probably due to increased reflex irritability.

Caffeine causes the tentacles and polypi to lose their tonus, and become relaxed, which is not the case with chloroform. Medusae anaesthetised with chloroform when put into a solution of caffeine also lose their tonus, but their irritability is restored, though their spontaneity is not.

The effects of strychnine differ in different species of medusae. In Sarsia it accelerates the rhythmical contractions which occur in groups separated by intervals of quiescence. This quiescence finally becomes continuous, and during it the animal does not respond to irritation of the tentacle, but does so to direct muscular stimulation.

Veratrine first increases the number and power of the contractions; afterwards it diminishes both.

Digitalin first quickens them, then renders them regular, causes persistent spasms, and produces death in strong systole.

Atropine causes first acceleration, then convulsions, then feeble contractions, and finally death in systole.

Nicotine causes violent and continuous spasm, with numerous minute rapid contractions superimposed upon it. These latter soon die away, leaving the bell in strong systole.

After spontaneous movements have disappeared, the bell no longer responds to stimulation of the tentacles, but responds to direct stimulation.

Alcohol first greatly increases the rapidity of the contractions, so much so that the bell has no time to expand properly between them, and they are in consequence feeble and gradually die out. The reflex stimulation shortly ceases to produce any effect, but muscular irritability is longer maintained.

Cyanide of potassium first quickens and then enfeebles the contractions; spontaneous movements rapidly cease, and the bell soon becomes irresponsive either to irritation of the tentacles, or to direct irritation. For a long time after it has become irresponsive, the nervous connections between the tentacles and polyp remain intact, as also do the nervous connections of these organs with all parts of the bell. The sensory organs are therefore not paralysed by this drug.

The effects of poisons on medusae were localised by Romanes in two ways. One way was to divide the medusa almost into two halves, connected only by a narrow strip of tissue. These halves were plunged into two beakers filled with sea-water, pure in one and poisoned in the other. The connecting strip rested upon the edges of the beaker. When curare was employed as a poison in this way, it was found to have an action similar to that which it exerts on mammals : apparently paralysing the motor nerves, while it left the sensory nerves capable of action. Thus, on nipping the half of a medusa which was plunged in the curare solution, it remained absolutely motionless, while the other half at once responded by a peculiar contraction to the stimulus. Here, also, however, just as in mammals, the sensory fibres are also paralysed by a large dose, so that if much poison be used, irritation of the poisoned part will have no effect either upon it or upon the unpoisoned part. When experimenting in this way with strychnine, Krukenberg found that the excitability of the poisoned part. was increased, so that when he touched the connecting strip lightly with a needle no effect was produced on the unpoisoned half, but the poisoned half responded by several energetic contractions. Veratrine had a similar action to that of curare, so that irritation of the poisoned half caused no movement in it, but caused movement in the unpoisoned half. The irritability of the contractile tissue is also diminished so that it no longer reacts so readily in the poisoned half to electrical stimuli.

Fig. 32.   Diagrammatic representation of the method of localising the action of poisons on medusae. One vessel contains normal sea water; another contains poisoned sea water, which is shaded in order to distinguish it.

Fig. 32. - Diagrammatic representation of the method of localising the action of poisons on medusae. One vessel contains normal sea-water; another contains poisoned sea-water, which is shaded in order to distinguish it.

Nicotine appears to paralyse the ganglionic structures and not the nerves.

It has already been mentioned that the rhythmical movements of medusae depend upon the ganglia: when these are all cut off the movements cease, but if only one be left the movements continue. In the medusa divided into two halves, as already described, it is evident that if the ganglia are removed from one half, or one half rendered functionally inactive by poison, that half will still continue to contract, so long as it remains connected with the other half, but will cease to move when it is completely divided from the half which still contains ganglia. The effect of nicotine is such as one would expect if the poison paralyses the ganglia, for it is found that when one half of a medusa is steeped in water containing nicotine, both halves still continue to pulsate rhythmically; so soon as the connecting band of tissue is divided, the poisoned half at once ceases to move, while the other half continues to pulsate.

The second way in which Romanes localised the action of poisons on medusae was by applying them to a strip of contractile tissue. He found that various poisons applied to the strip, or injected into it, caused a blockage of contractile waves, preceded by a progressive slowing of the rate of transmission along the poisoned part. Chloroform, ether, alcohol, morphine, strychnine, and curare, all have this effect.

General Results

The most marked results of experiments on medusae are, that the contractile tissue contracts rhythmically when stimulated by ganglia. It ceases to do so when the ganglia are removed and the contractile tissue left under ordinary conditions, but a constant stimulus, either chemical or electrical applied to it after the removal of the ganglia, will cause it to beat rhythmically just as if the ganglia were present. This appears to show that the rhythmical contractile power is a function of the contractile tissue and not merely of the ganglia. Besides its power of contracting once on the application of a single stimulus, or rhythmically from continued stimulation, the contractile tissue also possesses the power to conduct stimuli. This is seen in the passage of the contraction wave along a strip of medusa which, on reaching the bell, causes it to contract. When two contraction waves travelling along the contractile strip in opposite directions meet one another they arrest each other. This mutual extinction may be regarded either as a process of inhibition or interference, or as a consequence of exhaustion of the tissue which possibly may be unable to contract twice with such a short interval between.

The power of the contractile tissue to transmit stimuli is diminished or destroyed by cutting it more or less completely across, by compression, by stretching, by very high or low temperatures, and by poisons such as chloroform, morphine, nitrite of amyl, caffeine, strychnine, curare, and indeed almost any foreign substance added to the water in which the strip is immersed.

There are, however, two conducting channels, along which stimuli may be transmitted; the first, already mentioned, is the contractile tissue; the second is the nervous tissue. The passage of stimuli along the second is rendered evident by the movements of the tentacles. These nervous or tentacular waves and the contractile waves may exist either together or separately. The nervous waves are excited by stimuli which are too weak to excite contraction waves, and it is to be particularly remarked that when this is the case they only travel at half the rate at which a contraction wave travels, although, when the stimulus is strong enough to excite a contraction wave also, both the nervous and the contractile wave travel at the same rate, the nervous one being a little ahead of the other. The passage of nervous stimuli may also be diminished or completely blocked by section or compression just as in the case of contraction waves.

The transmission of stimuli along nerves is also affected by poisons. It appears to be destroyed by anaesthetics, though more slowly than that of the contractile tissue. The ganglia may be paralysed, e.g. by nicotine, before the transmission of nervous stimuli from them is diminished. The contractile tissue alone may be paralysed.