III. ANTHOZOA 



227 



In the Hexacoralla (fig. 194) the septa are in pairs, with the muscle ridges 

 facing each other, except at the ends of the sagittal axis, where they face out- 

 wards. These are called the directives. Since the septa occur in' pairs, two 

 kinds of radial chambers occur, those between the pairs being called inler- 

 septal, those between the two of a pair being intrascptal. At first all Hexac- 

 tinians have six pairs of septa two pairs of directives, and four of lateral 

 septa. With growth, septa of a secondary order may appear between these, 

 giving twelve in all, then tertiary septa, the number of tentacles increasing 

 with the septal chambers. The rule is not invariable, for some have modified 

 the plan of six to four or ten, without altering the primitive condition. 



A 



B 

 FIG. 194. FIG. 195. 



FIG. 194. Transverse section of actinian (Adamsia diaphana) AB, plane of sym- 

 metry, a second lies at right angles. I-IV, septa of four orders. 



FIG. 195. Transverse section of an Octocorallan (Alcyonium}. x, siphonoglyphe; 

 1-4, septa of one side, with their muscles on one side, symmetrical with those of the 

 other side. 



In the Octocoralla only eight septa are developed. These are disposed 

 equally on either side of the oesophagus and may have (most octocorallans) all 

 their muscles towards one end (fig. 195) or (Edwardsia, fig. 196, IV) have one 

 pair reversed. It is to be noted that hexactinians pass through an Edwardsia 

 stage. In Cerlanthus new septa are always added at one end of the sagittal 

 axis (fig. 196, II), while in the extinct Tetracoralla (I), so far as one may judge 

 from the hard parts, the septa have an arrangement with four as the basis. 



Most Anthozoa reproduce by division or budding as well as by eggs. 

 Occasionally the buds separate; usually they remain connected with the 

 mother, forming colonies of hundreds or thousands of individuals, con- 

 nected by a cccnosarc, consisting largely of mesogloea with a covering 



