; 



On Coral Reefs and Islands. 185 



in the July following, each had nearly reached the surface, and 

 was quite immovable; and some had grown over the others. 

 Mr. Stutchbury describes a specimen consisting of a species of 

 oyster, whose age could not be over two years, encrusted by an 

 Agaricia, weighing two pounds nine ounces.* It is stated by M. 

 Duchassaing, in a letter from Guadeloupe, that in two months, 

 some large individuals of the Madrepora prolifera^ which he 



^ nal size.f 

 Since the return of the Expedition, I have received a letter 

 containing some facts on the growth of Actinia? from Sir J. G. 

 Dalyell, whose able observations in this department of science 

 are highly curious and important. After speaking of the various 

 conditions and sizes of the young at birth, and of the difference 

 m the rapidity of growth depending on the amount of nutriment 

 at hand,^ he says, speaking of a Scottish species of Actinia, 

 ^^ The dimensions will generally double in a fortnight from its 

 birth, ^ The diameter of the base being originally about an eighth 

 of an inch, or hardly as much, will be five-eighths in six months, 

 and the tentacles will occupy a circle of an inch and a half in 



V diameter. In twelve or thirteen months, the diameter of the 



base will reach an inch and the expansion of the tentacles two 

 inches between the tips. An Actinia whose tentacula expanded 

 a quarter of an inch three weeks after it was produced, enlarged 

 so much in five months that they expanded an inch, and the 

 body was then half an inch thick," ^If we reason upon these 

 data, and assume that the Madrepore polyps may increase lineally . 

 m SIX months as much as the young Actinia, we shall have an 

 elongation of five-eighths or three- fourths of an inch in six months. 

 Taknig the still more rapid rate^ of doubling in a fortnight, 

 which might be more correct, since the Madrepore polyps are 

 ^bout the size of the Actinia in its earliest state, we should have 

 a lengthening of a fourth of an inch in a month, and three inches 



4 a year. The data upon which this conclusion is based, though 



important, are uncertain, but would probably give too high rather 

 than too low an estimate. And yet it is far below the rate ap- 

 parently established by the experiments with corals cited in the 

 preceding paragraph. We must admit that the subject requires 

 more accurate investigation. 



The stay of the ex^pedition near any particular reef in the Pa- 

 cmc was too short for any examinations by us. They might 

 easily be made by those residing in coral seas, either in the man- 

 ner adopted by Mr. Allan, or more definitely by placing marks 

 ^pon particular species. By insertirig slender glass pins a certain 

 distance from the summit of a Madrepore, its growth might be 



* West of Englund Journal, vol i, p. 60, 

 f Ulnstitut, No. 639, April 1, 184G, p. 111. 



Secoxd SiiEiEs, Vol. XII, N-a S5.— Sept., 1851. 24 



