Go down at low spring-tide to the nearest ledge of rocks, and with a hammer and chisel chip off a few pieces of stone covered with growing sea-weed. Avoid the common and coarser kinds (fuci) which cover the surface of the rocks; for they give out under water a slime which will foul your tank: but choose the more delicate species which fringe the edges of every pool at low-water mark; the pink coralline, the dark purple ragged dulse (Rhodymenia), the Carrageen moss (Chondrus), and above all, the commonest of all, the delicate green Ulva, which you will see growing everywhere in wrinkled fan-shaped sheets, as thin as the finest silver-paper. The smallest bits of stone are sufficient, provided the sea-weeds have hold of them; for they have no real roots, but adhere by a small disc, deriving no nourishment from the rock, but only from the water. Take care, meanwhile, that there be as little as possible on the stone, beside the weed itself. Especially scrape off any small sponges, and see that no worms have made their twining tubes of sand among the weed-stems; if they have, drag them out; for they will surely die, and as surely spoil all by sulphuretted hydrogen, blackness, and evil smells. Put your weeds into your tank, and settle them at the bottom; which last, some say, should be covered with a layer of pebbles: but let the beginner leave it as bare as possible; for the pebbles only tempt cross-grained annelids to crawl under them, die, and spoil all by decaying: whereas if the bottom of the vase is bare, you can see a sickly or dead inhabitant at once, and take him out (which you must do) instantly. Let your weeds stand quietly in the vase a day or two before you put in any live animals; and even then, do not put any in if the water does not appear perfectly clear: but lift out the weeds, and renew the water ere you replace them.