Vol. III. July, 1904. No. 2 



BIOLOGICAL BULLETIN' 



THE MORPHOLOGY OF THE MADREPORARIA. 

 V. SEPTAL SEQUENCE. 1 



J. E. DUERDEN. 



The skeleton of an ordinary poly cyclic hexameral coral pre- 

 sents a series of septal partitions arranged in a radiating manner, 

 and in any cycle the constituent septa are equal in size and alter- 

 nate in a regular manner with the members of the other cycles. 

 In general, the cyclic plan of a corallite is 6, 6, 12, 24, 48, etc., 

 and the number of septa in any cycle beyond the first corresponds 

 with the total number in all the cycles within. As the septa in 

 any cycle are alike in size it seems reasonable to suppose that they 

 all appeared simultaneously in the growth of a coral, a cycle at 

 a time, and also that the inner larger cycles were developed 

 before the outer smaller cycles ; in other words, the relative sizes 

 and positions of the septa in the mature corallite would seem to 

 represent their order of development. 



Much attention has already been given to the subject of the 

 order of appearance of the septa in corals by different writers. 

 The rule which Milne-Edwards and Haime give in their " His- 

 toire Maturelle des Coralliaires " (1857) is well known, appear- 

 ing in all text-books describing recent or fossil corals. The 

 authors assume that in the case of the first three cycles the con- 

 stituent septa of each cycle appear simultaneously, a cycle at a 

 time, and that the relative development of each cycle corresponds 

 with its relative size or distance from the center of the calice. 

 From the fourth cycle outwards, however, a different sequence is 



1 The first two parts of this series of papers appeared in the Johns Hopkins Uni- 

 versity Circulars, Vol. XXL, Nos. 155 and 157, and were reprinted in the Annals 

 and Magazine of Xatnral History, Ser. 7, Vol. X., May and August, 1902. The 

 third and fourth parts appeared in the Annals and Magazine of Natural History, 

 Vol. X., November, 1902, and Vol. XL, February, 1903. The work is being 

 carried out with the assistance of an appropriation from the Carnegie Institution. 



Contributions from the Zoological Laboratory of the University of Michigan. 

 No. 8l. 79 



