ACTINOZOA. MADREPORARIA. 53 



numerous calcareous partitions, the most important of 

 which are the septa (fig. 11 A, 6). These are vertical 

 plates running from the theca towards the centre, and 

 alternating with the mesenteries. They are of different 

 sizes, some reaching the centre, others going only part of 

 the distance ; they frequently occur in cycles, of which 

 three or more may often be distinguished, the largest 

 being the primary (6), the others the secondary, tertiary 

 and so on. In some forms one or more of the primary 

 septa are absent, and we have what are known as septal 

 fossulce. When the septa project upwards above the edge 

 of the theca, they are said to be exsert. Sometimes they are 

 only feebly developed, being represented by rows of spines. 

 When seen in section the septa are frequently found 

 to consist of three layers (fig. 11 B), the middle being 

 darker and thinner than the outer, and apparently repre- 

 senting the original septum, the outer layers having been 

 formed subsequently. In the centre of the visceral 

 chamber, where the larger septa come together, there is 

 often a vertical rod, which extends from the base of the 

 chamber to the bottom of the calyx, this is the columella 

 (figs. 11 A, a; 13 c). Its structure varies considerably; 

 when it is solid and ends in a knob or point in the calyx, 

 it is termed styliform ; if the top is porous, it is said to 

 be spongy ; when it consists of twisted laminae it is termed 

 trabeculate ; if formed by the twisting together of processes 

 given off from the inner edges of the septa, it is false ; 

 and lastly in some forms it is altogether absent. Another 

 set of vertical partitions somewhat similar to the septa are 

 the pali (fig. 11 A, c), these are radiating plates attached 

 to the columella and placed opposite the inner edges of 

 some of the shorter septa, but not joining them. Passing 

 from one septum to another, there are often rods known as 



