ON CORAL AND CORAL REEFS 121 



of multitudes of little hair-like filaments, called cilia, 

 with which its body is covered. These cilia all lash the 

 water in one direction, and so drive the little body along 

 as if it were propelled by thousands of extremely minute 

 paddles. After enjoying its freedom for a longer or 

 shorter time, and being carried either by the force of 

 its own cilia, or by currents which bear it along, the 

 embryo coral settles down to the bottom, loses its cilia, 

 and becomes fixed to the rock, gradually assuming the 

 polype form and growing up to the size of its parent. 

 As the infant polypes of the coral may retain this free 

 and active condition for many hours, or even days, and 

 as a tidal or other current in the sea may easily flow at 

 the speed of two or even more miles in an hour, it is 

 clear that the embryo must often be transported to very 

 considerable distances from the parent. And it is 

 easily understood how a single polype, which may give 

 rise to hundreds, or perhaps thousands, of embryos, 

 may, by this process of partly active and partly passive 

 migration, cover an immense surface with its offspring. 

 The masses of coral which may be formed by the as- 

 semblages of polypes which spring by budding, or by 

 dividing, from a single polype, occasionally attain very 

 considerable dimensions. Such skeletons are some- 

 times great plates, many feet long and several feet in 

 thickness ; or they may form huge half globes, like the 

 brainstone corals, or may reach the magnitude of stout 

 shrubs or even small trees. There is reason to believe 

 that such masses as these take a long time to form, and 

 hence that the age a polype tree, or polype turf, may 

 attain, may be considerable. But, sooner or later, the 

 coral polypes, like all other things, die; the soft flesh 

 decays, while the skeleton is left as a stony mass at the 

 bottom of the sea, where it retains its integrity for a 



