ANTHOZOA. ZOANTHARIA 79 



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 : in some genera there is no 

 columella. Other vertical partitions, somewhat similar 

 to the septa, are the pali (fig. 24 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. Bars or rods, known as synapticulce, are often found 

 passing from one septum to another. Similarly, adjacent 

 septa are often connected by plates, which may be horizontal 

 or oblique, straight or curved, and are called dissepiments 

 (figs. 24 e ; 31 d) ; in some genera they are very abundant 

 near the margin of the visceral chamber and form a 

 spongy or vesicular tissue (fig. 30 d). Lastly we have 

 the tabular (figs. 30 t; 31 B, t), which are more or less 

 horizontal plates, crossing the septa, and occupying the 

 central part of the visceral chamber, or, when well developed, 

 extending quite across it; they are arranged one above 

 another, so that the visceral chamber is divided into 

 horizontal compartments. On the outside of the wall of 

 the coral there are, in some forms, vertical ridges, which 

 may be smooth or spiny ; these are known as costaz, and 

 usually correspond in position with the septa. 



The young coral polyp is a free-swimming animal; 

 when it becomes fixed the first part of the skeleton to 

 appear is a circular plate between the base of the polyp 

 and the surface to which it is attached ; on this plate radial 

 ridges — the first traces of the septa — are secreted in folds 

 formed in the base of the polyp between the mesenteries. 

 The theca next appears at the edge of the plate, and is 

 formed either by the union of the ends of the septa, or as 

 an independent structure. For some time the polyp ex- 

 tends down to the base of the cup-like skeleton (fig. 22) 



