Page:Popular Science Monthly Volume 73.djvu/487

Rh

transmits them is almost certain to be more complete on one side than on the other of any stimulated point. Of course the waves meet as in Fig. 2, B, and then the strong wave destroys the weak one and continues around the ring. There is then only one wave left in the circuit and this travels constantly around (Fig. 2, C) for hours or days until something stops it. such as the cutting of the circuit or a fresh stimulus which produces a wave that meets and destroys it.

The weak wave was destroyed by the strong one in the above experiment because a weak stimulus can not set tissue into pulsation, which has been caused to pulsate through a strong stimulus, until after an appreciable interval of rest. Thus a weak stimulus following immediately after a strong one will produce no contraction, whereas a strong stimulus may cause tissue to pulsate even immediately after it has responded to a weak one.

It is now evident that the disk without its sense-organs can pulsate in sea-water if only a wave be once started in it, but that under normal conditions there is nothing to start a wave, and thus the disk remains quiescent. In other words, the sea-water is indifferent, and neither stimulates nor inhibits pulsation.

It is now time for us to determine why it is that the sea-water does not stimulate the disk when its sense-organs are removed. In the first place we must know the composition of sea-water, and chemical analysis shows that it consists of a mixture of sodium chloride (common salt), magnesium chloride and sulphate, potassium chloride, and calcium chloride and sulphate.

Numerous experiments show us that the common salt is a strong stimulant to both nerves and muscles. On the other hand, magnesium, calcium and potassium, all inhibit and do not stimulate the disk. Indeed, the stimulating effect of the common salt in the sea-water is exactly offset by the subduing tendency of the magnesium, calcium and potassium; and thus it is that the sea-water as a whole neither stimulates nor inhibits the pulsation of the jellyfish. The sea-water