50 



Papers from the Marine Biological Laboratory at Tortugas. 



S e.cL vvSL't 6r 



ICO sea. NA(Ltev -f \ o f^^ c^Ci ^^ .h- rnx 



oo sea wa.ter -V 20 nAc>cI 



J 2 



30^^ 



30 c 



loo sea ^a,-ter -v ^0 !V\^ C l-^ 



2 ^ 7 C 



Sea watier 



x% c 



Fig. II. 



Thus apart from osmotic effects the degree of concentration of mag- 

 nesium makes but little difference in stupefying marine animals by Tull- 

 berg's method of adding MgCl2 or MgS04 to the sea-water. The com- 

 parative unimportance of the magnesium ion may also account for the fact 

 that while the relative concentrations of the sodium, calcium, and potassium 

 in the blood of mammals remain pretty much as they are in sea-water, the 

 magnesium has declined from 3.7 to 0.4 or to about one-ninth its former con- 

 centration (see "The fitness of the environment," p. 187, by Laurence J. 

 Henderson, New York, 1913). 



Normal medusae soon cease to pulsate if placed in a solution of 100 c.c. 

 sea-water + 30 c.c. of 0.4 molecular MgCl2, and they may remain in this 

 solution for 8 days without further pulsation, the bell, however, slowly 

 disintegrating, due to bacterial action which the weakened concentration 

 of sodium is unable to check. Whenever these magnesiumized medusae 

 are replaced in normal sea-water they at once go into clonic tetanus and then 

 pulsate normally. This initial tetanus resembles that which develops in 

 sea-water lacking magnesium, and is there due to the combination of sodium 

 and calcium unchecked by magnesium. It seems, then, that an excess of 

 magnesium acts as a shield or "buffer" to prevent the calcium from pro- 

 ducing tetanus, and when this shield is suddenly removed the medusa is 

 unable to withstand the effects of even the normal concentration of calcium.* 

 It will be recalled that Mines (191 1 b, p. 185) arrested the pulsation of the 

 heart of a ray by magnesium and then restored pulsation by simply raising 



>S6rensen, 1909, Comptes Rendus Lab. Carlsberg, vol. 8, p. i, calls attention to the action of various 

 sorts of "buffer" substances in physiological solutions. 



