The most curious of the tube-building Rotifers are those which form their tubes of little, equal-sized pellets of solid matter–as it were, "bricks"–which they first form by compacting fine particles in a special pit on the head and then build them up and cement them together in rows to form the tube, adding row after row as the animal itself increases in length (Fig. 37 (_bis_), C). These are known as Melicerta; and, though some kinds use any minute particles to make their bricks, one kind is frequent which uses its own excrement for this purpose. By feeding the little creatures first with food coloured with carmine and then with blue-stained material, one can obtain alternate rows of pink and blue pellets, carefully manufactured and laid in position to build up the growing length of tube. Melicerta has certainly an extraordinary and economical way of disposing of that refuse which we larger creatures carefully remove from our habitations and should be very unwilling to employ as building material. The individuals of one rare and interesting kind of the tube-builders, after swimming freely in the youngest stage, settle down together and form their gelatinous transparent tubes side by side, to the number of fifty or more, in such a way as to produce a perfect sphere, a twentieth of an inch or more in diameter, built up of fused jelly-like tubes radiating from a common centre. The inhabitant of each tube is quite separate from and independent of his neighbours, but they all protrude their vibrating wheel-apparatus simultaneously, and cause the glass-like ball to rotate and travel through the water. Many years ago I found this beautiful little thing in a small moss-pool (not more than 3 ft. wide), high up the sloping-side of the north-west section of Hampstead heath, above the "Leg of Mutton Pond." The well-meant care of the public guardians of the heath has now drained this region, and my little moss-pools and the "bog," in which grew the Drosera, or Sun-dew, and the Bog-bean and such plants, have gone for ever. But we must console ourselves with the fact that the same progressive expansion of the great city has given us electric railways, tubes, and tramways by which we can go farther afield than Hampstead in a few minutes, and still find moss-pools and the undisturbed glories of ancient swamps and bog-land.