MADREPORARIAN CORALS 93 



If, now, the terminal branch of the commoner type of 

 Madrepora be broken off and the surface of the fracture be 

 examined, it wiU be found that in the axis of the branch 

 there is a cavity traversed by radial septa which are con- 

 tinuous with the septa of the apical calyx. 



It follows from this observation that the terminal branch 

 represents the elongated calyx of a polyp which has given 

 rise, by centrifugal gemmation, to a number of lateral polyps. 

 The growth and fusion of the walls of the lateral polyps 

 completely enshroud the calyx of the apical polyp which has 

 given birth to them, and there is no common substance or 

 coenosteum between them. 



The number of septa in the lateral calices is usually six, 

 and of these the two directive septa situated in planes which 

 are radial to the axis of the branch are decidedly larger than 

 the others and frequently meet in the centre of the calyx 

 (Fig. 6, p. 32, DS). There is no columella. The apical 

 calices have usually more than six septa, and in some cases 

 the lateral calices have also more than six septa, but the 

 number of septa seems to have reached its maximum when 

 twelve have been formed, and calices with more than twelve 

 septa are very rarely found. The character of the endotheca 

 is very variable, but it is noteworthy that in some cases it 

 takes the fomi of more or less regularly disposed tabulae. 



The polyps of an expanded Madrepora appear to be of 

 two distinct kinds. The apical polyps, projecting some 

 3 mm. beyond the apex, of the branch, have only six long 

 tentacles, and the lateral polyps, which project very little 

 beyond the lip of the calyx, have twelve tentacles, six long 

 and six short. But this does not seem to represent a true 

 dimorphism such as we find in the polyps of many of the 

 Hydrozoa and Alcyonaria, because at the rapidly growing 

 margins of the colony many intermediate forms between 

 the two kinds may be found, and it seems probable that a 

 polyp of the twelve-tentacled kind may change into a polyp 

 of the six-tentacled kind when it assumes the function of an 

 apical polyp and starts the development of a new branch. 



The tentacles seem to vary a good deal in character. 

 Sometimes they are simply digitiform, sometimes they 



