But these, beautiful as they are, are nothing, so far as mass and dominating vigour of growth are concerned, in comparison with the reef-building corals of the warm seas of the tropics. There these lime-secreting conglomerated sea-anemones separate annually hundreds of tons of solid calcite per square mile of sea bottom from the sea-water, and build up reefs, islands, and huge cliffs of coral rock. They get the calcite--as do calcareous seaweeds and shell-making clams, oysters, 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, belonging to different classes, which are glass-like in transparency and colourless--so as to be nearly or quite invisible in clear water, and some, too, occur in fresh waters (larvæ 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 surface by the alternate contraction and expansion of its disk-like body, and then dropping gently through the full length of the jar to the bottom, when it would again mount. On the downward journey--owing to its transparency--it would encounter unsuspecting, jerkily-moving water-fleas, unwarned by any shadow cast by the impending glass-like monster of half an inch in breadth slowly approaching from above; and as soon as they touched it they were paralysed (by microscopic poison-threads like those of the sea-anemones), and were grasped and swallowed by the mobile transparent proboscis (like that of an elephant, though certainly smaller, and