42 James D. Dana, Esq., on the Structure^ 



surface is gorgeously decked with polyp stars of purple and 

 emerald green.* 



All the many shapes proceed in each instance from a single 

 germ, which grows and buds under a few simple laws of 

 development, and thus gives origin either to the branch, the 

 broad leaf, the column, or the hemisphere. 



e. Life and Death in Concurrent Progress. — But the more 

 massy forms would not exist, and others would be of dimi- 

 nutive size, were it not for a peculiar mode of growth which 

 characterises most coral zoophytes. 



Life and death are here in concurrent or parallel progress, 

 a condition favoured by the existence of coral secretions. In 

 some instances, a simple polyp, while growing at top and 

 constantly lengthening itself upward, is dying at its lower 

 extremity, leaving the base of the coral bare, and destitute 

 of any living tissues. The polyp thus continues rising in 

 height, and death progresses below at the same rate, till 

 at last the live polyp may be at the extremity of a coral 

 stem many times its own length. This process is illustrated 

 by figures on pages 62 and 78 of the Report on Zoophytes. 



In species which bud and form large groups, the same ope- 

 ration takes place. In some instances the summit polyp or 

 polyps bud and grow, while at a certain distance below the 

 summit, the work of death is going on and polyps are gradu- 

 ally disappearing. There is thus a certain interval of life, 

 the length of which interval is different for different species. 

 There are zoophytes which grow to a height of several feet, 

 and still only the upper one or two inches are living. The 

 recent polyps at the top of the column are active with life, 

 and vigorous in reproduction, while the more aged below, 

 having reached the fixed limits of their existence, are disap- 

 pearing. The enduring coral remains and constitutes the 

 basement or stage of action for future generation of polyps. 

 But this death is not in progress alone at the base of the 

 column or branch. Generally the whole interior of a corallum 

 is dead, a result of the same process with that just explained. 

 Thus, a Madrepora, although the branch may be an inch in 



* See Dana's Report on Zoophytes, pp. 29 and 69-61. 



