170 SECRETS OF EARTH AND SEA 



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 posi- 

 tion to build up the growing length of tube. Melicerta 

 has certainly an extraordinary and economical way of dis- 

 posing of that refuse which we larger creatures carefully 

 remove from our habitations and should be very unwilling 

 to employ as building materal. The individuals of one 

 rare and interesting kind of the tube-builders, after swim- 

 ming 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 

 protude 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 pro- 

 gressive 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. 



Many of the Rotifers have a pair of ruby-red eyes, and 

 in some of them there is a minute crystalline lens over- 



