80 CORAL AND ATOLLS 



Turning now to the corals that constitute the second class, 

 and have some of their units specialised as active agents of 

 growth, it is at once seen that the possibilities of variation 

 of normal vegetative habit are greatly increased. All the 

 elaborate branching forms, plates, and leaf-like growths belong 

 to this class ; and all are evolved by special peculiarities of the 

 growing point. The zooids that constitute the growing point 

 may take various forms : they may be arranged as a cluster, as 

 a creeping edge, or as many varieties of terminal shoots of 

 branches. 



In the first instance, it is necessary to draw very sharp 

 distinctions between two subdivisions of this group. In 

 Growp 1 come all those forms like Monti^pora, whose distal 

 zooids are the newest formed members of the colony ; and in 

 Group 2 are included the Madrepora, whose distal zooid is the 

 most ancient individual in the whole growth. 



In dealing with Group 1 many forms have to be considered, 

 for when the youngest are the active zooids their growth-cluster 

 may be very variously disposed, and on its disposition the 

 resulting vegetative form entirely depends. 



When the growing zooids are arranged in linear series, a flat 

 growth will result, which grows from one of its edges, or from 

 them all ; and in this way an encrusting layer or a free plate 

 may be formed. Corals that grow with a linear growing- 

 point may settle down on a basis, and spread over it in all 

 directions, taking an exact impression of every irregularity of 

 its contour ; and then, reaching the limits of the basis, they 

 may grow from it as flat plates, spreading out from its margins. 

 Such corals as grow in this fashion show many curious changes 

 at their free margins, for although the superior surface of the 

 encrusting layer can alone produce zooids, still at the free 

 edge zooids will appear on both superior and inferior surfaces. 



In connection with these partial plates a very curious fact 

 is always demonstrated, for the whole surface structure of the 

 coral body, and of the corallites, differs above and below ; and 

 this important fact will need further reference. 



A coral may start from its first beginning by growing as free 



