i6o THE SEAS 



Mediterranean and in the coral reefs of the tropics. They 

 never form so thick a mass as do the true corals, being 

 essentially encrusting plants forming a compact layer over 

 any hard surface, rocks, stones or dead coral. An im- 

 portant green weed which also deposits carbonate of lime 

 in its fronds and is so common as to form an important 

 constituent of many coral reefs, is called Halimeda. It has 

 a characteristic appearance being formed of a number of 

 broadened, limy lobes united by narrow, uncalcified joints 

 which make the plant relatively flexible. 



Having indicated the chief agencies concerned in the 

 formation of reefs, we must now turn our attention to their 

 manner of growth and life in general, because, as we shall 

 see later, these are of the very first importance in the 

 formation of coral reefs. The great coral colonies develop 

 by a process of budding and division from the original 

 parent individual, in essentially the same manner as a plant 

 sends out new shoots and branches, a method of increase 

 which is known as vegetative propagation. Going back 

 to the beginning of things, the young corals are incubated 

 within the body of the parents for a little time, but at a 

 comparatively early age the " planulae " escape and swim 

 about in the sea by means of a coating of fine hairs with 

 which chey are provided. At this stage they are minute, 

 pear-shaped bodies no bigger than a pin's head. After a 

 time they settle down on a convenient hard surface, the 

 under surface spreads out to form a broad basal disc while 

 the upper surface is pushed in to form a mouth and then 

 a stomach cavity, while around the mouth grow out the 

 tentacles. 



The skeleton now begins to form and the individual 

 grows to its full size encased both below and on all sides 

 by a thick layer of carbonate of lime. If it is a solitary 

 coral, it has now completed its development and has in 



