ADULT COLONY. 17 



are very rare, but occasionally much enlarged calices are seen undergoing 

 binary fission. The detailed study of the development of young bud polyps 

 has not been carried out, as the species is unsuited for the purpose. It has 

 been established, however, that the manner of appearance of the tentacles, 

 mesenteries, and septa agrees closely with that of larval polyps to be 

 described later. 



In what seem to be dead parts of a colony, new polyps may arise 

 within the old calices. Outwardly these polyps are at first wholly sepa- 

 rated from one another and from the other living tissues of the colony, and 

 are usually transparent and colorless, due to an absence of Zooxanthellae. 

 Similar renewals of growth have been found in other corals, and give rise to 

 the successive deposits of skeletal matter on dead, corroded surfaces, some- 

 times met with when masses of coral are broken across. The continuity of 

 any one corallite is frequently maintained throughout the vertical extent of 

 the mass, notwithstanding the corroded surfaces here and there. 



The renewal of the polyps within old corroded calices of a coral is a 

 subject for further investigation. So far as could be made out from external 

 observation alone such polyps in Siderastrea are altogether independent of 

 one another, and also unconnected with other living tissues of the colony. 

 It may be that within the deepest parts of the old calices there still remained 

 living remnants of the original polyps, and that from these new polyps were 

 formed as buds ; on reaching the same size as the original polyps the buds 

 would fuse with one another by their peripheral borders and present a 

 continuous covering of soft tissues. New skeletal deposits would cover 

 over the old surface, and the presence of the latter would afterwards be deter- 

 minable only on breaking the colony across. 



Renewals of this character, by the production of independent buds, are 

 to be distinguished from the growth at the living margin of colonies which 

 is frequently seen encroaching over dead areas. The latter is of the same 

 character as ordinary marginal gemmation from the parent stock, even 

 though it extends over old corroded surfaces. Such new growth is very 

 frequent in branching stocks of Acropora, but may occur in almost any 

 colonial species. 



The death of the polyps over any restricted area of a colony may be 

 brought about by many causes, such as the colony becoming partly covered 

 by or embedded in sand or mud, or by adherence or contact with other 

 foreign bodies, or by exposure. Local death is very frequent in such large 

 hemispheroidal colonies as those of Mtzandrina and Orbicella ; the middle 

 portion of the blocks may be dead and disintegrating while the sides are in 



