34 PULSATION OF JELLYFISHES. 



caused a paralysis of the marginal sense-organs , but not of the sub- 

 umbrella tissue of the disk. 



This we can prove directly, for disks without sense-organs, once 

 they be set into pulsation, will continue to pulsate for over three hours 

 in a solution resembling sea- water but lacking calcium . The amplitude 

 of their pulsations, however, decreases steadily, but may be restored 

 by adding calcium to the solution. It is evident that the central parts 

 of the sub-umbrella of Cassiopea may pulsate both in normal sea- water, 

 and for a long time in sea- water deprived of calcium, whereas the mar- 

 ginal sense-organs are quickly paralyzed by a deficiency of calcium 

 in the sea- water. On the other hand, perfect Medusae and disks 

 deprived of sense-organs will pulsate in sea- water at 82 F. containing 

 CaSO 4 + CaCO 3 to saturation, the only effect being a slight slowing 

 of the rate of pulsation in the case of the perfect Medusae. Hence the 

 marginal sense-organs require calcium* to perform their function, 

 whereas the general tissue of the sub-umbrella is relatively unaffected 

 by the presence or absence of calcium. This is, however, a relative 

 matter, for while the lack of calcium produces less effect upon the disk 

 than upon the sense-organs, nevertheless the disk itself will finally 

 cease to pulsate in the absence of calcium. It is interesting to observe 

 that while the disk is almost unaffected by a wide range in the amount 

 of calcium in the sea- water, it is very quickly affected by a change in 

 the amount of the potassium. Such disks cease to pulsate in a few 

 minutes either in a solution resembling sea-water but lacking potassium 

 or in a solution of % gram K 2 SO 4 in 100 c.c. of natural sea-water. 

 Indeed, the center of the disk is fully as sensitive to changes in the 

 amount of potassium in the water as is the entire Medusa. 



Under normal conditions pulsation is controlled by the marginal 

 sense-organs, the rate being that of the fastest working sense-organ. 

 The general sub-umbrella surface has considerable influence in sus- 

 taining the sense-organs, for if we reduce the area of the sub-umbrella 

 enervated by the sense-organs the rate declines. Normally the pul- 

 sation is controlled by the sense-organs, not by centers of pulsation in 

 the undifferentiated sub-umbrella tissue. Among thousands of nor- 

 mal Medusae I observed only two individuals in which a center in the 

 sub-umbrella controlled the pulsation. These two were pulsating 

 slowly when I lifted them out of water and threw them forcibly back. 

 They instantly began to pulsate in the rapid, uniform, clockwork-like 

 manner characteristic of pulsation maintained by a center in the sub- 

 umbrella, their rates being fully four times as great as the normal. I 

 then cut off their marginal sense-organs, and the disks still continued 



*The chief role of calcium is to counteract the anesthetic effects of magnesium. 



