THE SPECIES OF THE GENUS MILLEPOBA. 137 



of other corals. Its form must be adapted to the space left between its 

 neighbours on the crowded reef. Again, its form must be modified by 

 the depth of the water in which the embryo happens to develop. As 

 Duchassaing and Michelotti pointed out long ago, Millepora often 

 grows in very shallow water and is consequently unable to develop in 

 height. Specimens that happen to fix themselves on foreign bodies on 

 the edge of the reef at a depth of 5 or 6 fathoms can and do grow to a 

 very great length without impediment. 



It is also extremely probable that the available food supply, the par- 

 ticular set of the tides and currents, and the chemical composition of 

 the sea-water, particularly as regards the amount of calcium carbonate 

 it holds in solution, vary very considerably in different reefs and in 

 different parts of the same reef. Such variations must affect the rate 

 of growth of Millepores, and I think it is reasonable to believe the 

 mode of growth also. 



(2) The Size of the Pores. — Dana, Milne-Edwards and Haime, and 

 Quelch have used the size of the pores as a specific character, but, with 

 one exception, to be referred to presently, they give no measurements, 

 being contented to use the expressions "very small," "large," 

 "minute," &c. Unless the zoologist has an immense number of speci- 

 mens from different localities to compare one with another, it is diffi- 

 cult for him to understand what is meant by such expressions ; but 

 even the naturalists of the great national collections would be mystified 

 by the case of M. alcicomis, whose gastropores are, according to 

 Quelch, very large, and according to Milne- Edwards and Haime, " tres 

 petits." I have measured a very large number of gastropores, taking 

 for each specimen an average of 6 or 12. 



The greatest average diameter of the gastropores I have found is 

 0"37 mm., the smallest is 0T3 mm., so that the difference between 

 those pores which might legitimately be called " very large," and 

 those that are " very small " is 0*24 mm. But these " large " pores 

 are very rarely seen ; the great majority of the gastropores are 

 between 0*3 mm. and 0*2 mm. This general result agrees fairly 

 well with the only measurement I have been able to find in the 

 literature of the subject, namely, that of M. murrayi by Quelch, which 

 is given as 0"25 mm. 



The question that had next to be considered was whether there is 

 any other feature constantly associated with large pores and with small 



