250 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 



an exposure of ten, and generally survive an ex- 

 posure of live. But altliough they thus continue 

 to live for an indetinite time, their vigour is con- 

 spicuously and permanently impaired ; while in 

 the fresh water irritability persists for a short time 

 after spontaneity has ceased, and the tentacles and 

 manubrium are strongly retracted. 



" Turning now to the case of the fresh-water 

 species, when tirst it is dropped into sea-water at 

 85° there is no change in its movements for about 

 fifteen seconds, although the tentacles may be re- 

 tracted. But then, or a few seconds later, there 

 generally occurs a series of two or three tonic 

 spasms, separated from one another by an interval 

 of a few seconds. During the next half-minute the 

 ordinary contractions become progressively weaker, 

 until they fade aw^ay into mere twitching convul- 

 sions, which affect different parts of the bell irregu- 

 larly. After about a minute from the time of the 

 first immersion all movement ceases, the bell re- 

 maining passive in partial systole. There is now 

 no vestige of irritability. If transferred to fresh 

 water after five minutes' exposure, there immediately 

 supervenes a strong and persistent tonic spasm, 

 resembling rigor mortis, and the animal remains 

 motionless for about twenty minutes. Slight 

 twitching contractions then begin to display them- 

 selves, which, however, do not affect the whole bell, 

 but occur partially. The tonic spasm continues 

 progressively to increase in severity, and gives the 

 outline of the margin a very irregular form ; the 

 twitching contractions become weaker and less fre- 



