CORAL-MAKERS AND JELLY-FISH 91 



whelks, and microscopic chalk-makers — from the sea 



the water of the sea which always has it ready in solution 

 for their use. And the sea gets it from the rivers and 

 streams which wear away and dissolve the old limestone 

 deposits now raised into mountain chains, as well as by 

 itself dissolving again in due course what living creatures 

 have so carefully separated from it. Sea water or fresh 

 water with a little carbonic acid gas dissolved in it 

 dissolves limestone and chalk — it becomes what we call 

 " hard." Neutralize the dissolved carbonic acid (as is 

 done in the well-known Clark's process for softening 

 water), and down falls the dissolved calcite as a fine 

 white sediment. These alternating processes of solution 

 and " precipitation " are always going on in the waters 

 of the earth and sea. 



The name "jelly-fish " has reference to the colourless, 

 transparent, soft, and jelly-like substance of the bodies of 

 the animals to which it is applied. There are a number 

 of marine animals, besides the common jelly-fish, belong- 

 ing to different classes, which are glass-like in trans- 

 parency and colourless — so as to be nearly or quite 

 invisible in clear water, and some, too, occur in fresh 

 waters (larvae of gnats, notably of the plume-horned gnat 

 Corethra). The transparency of these animals serves 

 them in two different ways — some are enabled by it to 

 escape from predatory enemies ; others, on the contrary, 

 are enabled to approach their own prey without being 

 observed. The latter was obviously the case with the 

 little fresh-water jelly-fish which appeared in great 

 abundance some years ago in the lily tank in Regent's 

 Park. The water was full of small water-fleas (minute 

 Crustacea), and the little jelly-fish, if removed from the 

 tank and placed in a tall glass jar filled with the tank 

 water, spent its whole time in swimming upwards to the 



