OF THE ACxVLEPII^ OF NORTH AMERICA. 



229 



jjreater part of tlic year in the more or less torpid state of egg-life, next grou' into a 

 polyp-like vegetation, which at a particular period produces a kind of bud ; and this, 

 flower-like, developes into a more highly organized structure, which is at last freed, and 

 moves about independently. These animal flowers, as it were, (and the comparison to a 

 flower is so striking, that, even now we know that there is nothing of a vegetable char- 

 acter in these animals, we cannot help using the term, as best expressing the impres- 

 sions we derive from the sight of such living beings,) — these flowers move about for 

 a certain time, until they are actually killed by the heavy showers of summer rain, — 

 another analogy between the development of these animals and the growth of some 

 plants. 



Medusae, of this species in particular, are very sensitive to the density of the medium 

 in which they live, and the mere change arising from the difference in density between 

 fresh water and salt water is sufficient to kill them almost instantaneously. Taking up in 

 a spoonful of sea-water a fresh Sarsia in full activity, when swimming most energetically, 

 and emptying it into a tumbler full of fresh water of the same temperature, the little animal 

 will at once drop like a ball to the bottom of the glass, and remain for ever motionless, 

 killed instantaneously by the mere difference of the density of the two media. This little 

 experiment, which I occasionally repeated at different intervals during the past summer, 

 has led me to the discovery, that the total disappearance of the small Medusae from Bos- 

 ton Bay uniformly coincides with the heavy rains, while the larger species survive. 

 Sarsia, Hippocrene, Tiaropsis, Pleurobrachia, Bolina, indeed all the small species 

 known to occur here, disappear entirely after heavy rain-storms, a few stragglers only, 

 which were probably moving at greater depths during the rain or concealed under more 

 sheltered places, being found afterwards. These also disappear entirely at the next fall 

 of fresh water. These little Medusas occur in large numbers, swarming near the surface 

 of the water, moving rapidly in all directions with the greatest freedom and energy. 

 They are exceedingly voracious, and feed upon any kind of marine animals, not sparing 

 their own species. 



When kept in confinement, the water in which they are preserved should be changed 

 frequently ; but when kept in good condition they will live in captivity for a longer time ; 

 and when placed in large glass jars, they can be watched in all their movements, and 

 very satisfactorily studied. They are, however, so transparent, that, to observe them 

 successfully, it is necessary to place them towards the light, or to watch them at night 

 with a candle behind the glass jar, when nothing about them can escape the attention 

 of the observer ; as the inner cavities, the tubes communicating with them, the progress 

 of digestion, and circulation of the digested food, can be traced as perfectly as their gen- 



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