42 PULSATION OK 



of combinations of potassium with magnesium and calcium, are pow- 

 erful stimulants to the disk of Cassiopea, causing strong but temporary 

 contractions. Repeated touches of a crystal of K 2 SO 4 to any one spot 

 on the sub -umbrella of Cassiopea soon renders the place insensitive to 

 further stimulation of any sort. For example, a single spot upon a 

 disk, deprived of sense-organs, was touched 17 times, in rapid succes- 

 sion, with a crystal of K 2 SO 4 and each time a contraction resulted. 

 The next 2 touches, however, gave no contractions; then followed 2 

 touches with contractions, 7 without contractions, i with, and finally 

 ii without contractions, etc. 



If normal Cassiopea with sense-organs intact be placed in sea-water 

 + 0.125 to 1.55 per cent K 2 SO 4 , KC1O 3 , KC1, or K 2 CO 3 they immedi- 

 ately pulsate at an abnormally high rate, but the movement soon 

 loses force, and the disk comes to rest expanded with the mouth-arms 

 strongly contracted. Medusae in 0.125 P er cent excess of K 2 SO 4 will 

 pulsate quickly at first and then more and more slowly, so that at 

 the end of 1 3 hours their rates are only about half the normal rate in 

 sea-water. On the other hand, Medusae in sea-water + 1.55 per cent 

 K 2 SO 4 will pulsate with great activity for a few moments, but will 

 cease all movement in less than 4 minutes. Also, a solution of K 2 SO 4 

 isotonic with the NaCl of sea-water at once reduces the rate of pulsa- 

 tion of normal Medusae and quickly brings them to rest without an 

 initial display of excitement. It appears that a small excess of potas- 

 sium acts as a temporary stimulus, whereas a large excess at once 

 inhibits pulsation. It is possible that the initial stimulation is due 

 to the physiological reaction of the tissues against the injurious 

 effects of the potassium. Temporary activity is commonly called 

 forth in animals by sudden injurious stimuli. In this connection it 

 is interesting to see that Carlson (1906) finds that potassium is a 

 primary stimulant for the heart of Limulus, but its action is quickly 

 followed by depression. 



An excess of i per cent potassium in the sea-water quickly lowers 

 the rate of movement of the arms of Lepas, causes tetanus-like con- 

 traction, and may be fatal in 10 minutes. 



The effect of potassium upon the disk without marginal sense- 

 organs is, however, different from its effect upon the normal, perfect 

 Cassiopea , for disks without sense-organs are actively stimulated into 

 pulsation for a short time in all excess of potassium from sea-water + 

 0.25 per cent K 2 SO 4 to a pure solution of K 2 SO 4 , or KC1, isotonic 

 with the NaCl of sea -water. Perfect Medusae, however, show no 

 increase in rate of pulsation in isotonic K 2 SO 4 , but steadily decline. 

 It seems probable, therefore, that a strong excess of potassium impairs 



