MORPHOLOGY OF THE MADREPORARIA. 33 



retians the bilaterality of development of its septa ; while in 

 the other the bilateral symmetry of growth becomes replaced 

 by a radial disposition of the parts. Whenever alar fossulre are 

 present they represent an incompletion in the establishment of 

 the newer septa of the alar region, as compared with species in 

 which no alar fossulae are represented ; they have only a develop- 

 mental significance, and would not correspond with any structural 

 peculiarity of the fully developed polyp. As would naturally be 

 expected from such an explanation even individuals of the same 

 species may vary much with regard to the presence or absence 

 of alar fossulae. In the calice of some specimens of Microcyclus 

 discus, for example, there is a distinct grouping of the alar septa, 



FIG. 6. A corresponding second pair of septa T (<J>) has appeared within the ventro- 

 lateral chambers. 



while in others the septa have become perfectly radial, and no 

 alar region is distinguishable. 



The alar fossulae represent regions where new septa are being 

 added, and the interspaces may reasonably be expected to be 

 different from the other septal interspaces, and likewise to vary 

 at different stages according as the new septa are just appearing 

 or are nearly fully developed. 



From developmental studies the conclusion is thus reached 

 that in all ordinary species of zaphrentoid corals the septa of the 

 alar region pass through a fossular stage as here understood, 



