112 THE ELEMENTARY NERVOUS SYSTEM 



tion of the bell is quiescent. Loeb (1900) has shown that 

 if the central portion of the bell of the craspedote medusa 

 Gonionemus is thus rendered quiescent, its activity may 

 be revived by placing it in a solution of % molecular so- 

 dium chloride, which is about isotonic with seawater. If 

 to this solution a small amount of calcium chloride or 

 potassium chloride is then added, the pulsations cease as 

 in ordinary seawater, for the solution is thereby rendered 

 a balanced one, as seawater naturally is, in that the stim- 

 ulating effects of the sodium are counteracted by the cal- 

 cium or the potassium. 



Such a quiescent edgeless bell has upon its subum- 

 brellar surface a nerve-net and a muscle layer. Whether 

 in Gonionemus the sodium solution stimulates one or both 

 of these has not been definitely determined, but in Rhizo- 

 stoma a partial answer to this question has been found by 

 Bethe (1909). When the eight marginal bodies of this 

 jellyfish are removed, its bell is quiescent in ordinary sea- 

 water. Under such circumstances, however, it can be 

 made to contract by stimulating it electrically or chem- 

 ically. If now a preparation is cut from a Rhizostoma 

 so as to include one marginal body and two muscle fields, 

 the latter separated from each other by a partial cleft that 

 allows the two lobes thus formed to be in physiological 

 connection only through their nerve-net (Fig. 35), it is 

 possible to determine the relative sensitiveness of the 

 marginal body as compared with that of the rest of the 

 neuromuscular mechanism. When in such a preparation 

 the lobe without the marginal body is immersed in a 0.62 

 molecular solution of sodium chloride and the lobe with 

 this body is in seawater, the rate of contraction of the 

 whole preparation remains constant and normal. If, how- 

 ever, the lobe with the marginal body is now immersed in 



