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



ever, live as long as other specimens. In many 

 cases Savsiie will revive even after ten minutes' 

 exposure; bat tlie time required for recovery is 

 then very long, and the subsequent pulsations are 

 of an exceedingly feeble character. I never knew a 

 specimen survive an exposure of fifteen minutes.* 

 In not a few cases, after immersion in fresh water, 

 the animal continues to pulsate feebly for some little 

 time ; and, in all cases, irritability of the contractile 

 tissues peisists for a little while after spontaneity 

 has ceased. The opalescence above referred to 

 principally affects the manubrium, tentacles, and 

 margin of the nectocalyx. While in fresh w^ater 

 the manubrium and tentacles of Sarsia are strongly 

 retracted. 



Thinking it a curious circumstance that the mere 

 absence of the few mineral substances w^hich occur in 

 sea-water should exert so profound and deadly an 

 influence on the neuro-muscular tissues of the 

 Medusa?, I was led to try some further experiments 

 to ascertain whether it is, as Agassiz affirms, to the 

 mere difference in density between the fresh and 

 the sea Avater, or to the absence of the particular 

 mineral suljstances in question, that the deleterious 

 influence of fresh water is to be ascribed. Although 



* The covered-eyed MedusiB survive .a longer immersion than 

 the naked-eyed — Aiirelia aurita, for in.stauce, requiring from a 

 quarter to half an hour's exposure before being placed beyond 

 I'ecovery. Moreover, the cessation of spontaneity on the first 

 ioimersion is not so sudden as it is in tho case of the naked-eyed 

 Medusae — tlie pulsations continuing for about five minute?, during 

 which time they become weaker and weaker in so gradual a 

 rnauuer that it is hard to toll exactly when they first ceage. 



