ANTHOZOA. ZOANTHAMA 85 



are introduced and are joined to the cardinal sides of the 

 previously formed septa. As growth proceeds all the later 

 septa (a — c), unlike the primary septa, gradually move 

 towards the counter septum, as indicated by the arrows 

 in fig. 29 G. 



In the adults of some Rugose corals (fig. 34 A) the 

 arrangement of the septa is similar to that just described, 

 so that on each side of the cardinal fossula and on the 

 counter side of each alar septum the later septa (metasepta) 

 have a pinnate arrangement. In other genera, however, 

 the pinnate plan is not seen in the adult (figs. 32, 33), since 

 all the septa either become free at their inner edges or 

 unite only at the centre of the coral ; and in such cases, 

 unless a fossula is present, the symmetry of the coral is 

 nearly or quite radial. 



From the description of the septal development given 

 above, it will be seen that the fossulse are breaks in the 

 sequence of the septa. The cardinal fossula (fig. 34 A, a) 

 is limited by the later septa added on each side of the small 

 cardinal septum. The counter fossula, on the opposite side, 

 where no new septa are introduced, is bounded by the two 

 primary septa (d, d) which enclose the counter septum (6). 

 The alar fossulse are the spaces between each alar septum 

 (c) and the newer septa which have been added on its 

 counter side. 



The fossilise have been regarded as pits or chambers for 

 those mesenteries which alone were specialised for repro- 

 duction. Another explanation of the nature of the 

 cardinal fossula is that it is due to the presence of a groove 

 on the ventral side only of the stomodaeum, similar to that 

 found in the living family Zoanthidse ; it is thought that 

 such a groove would account for the small size of the 

 cardinal septum. 



