INVERTEBRATA 



CHAP. 



three small knobs which will later coalesce so as to form the columella. 

 The columella is the median pillar which projects upwards from the 

 base of the theca, indenting the base of the polyp. The primary septa 

 become extremely thick where they join the thecal wall. Still later, 

 cycles of tertiary septa make their appearance, and from the edges of 

 these structures isolated pillars become separated which form the pali. 

 The skeleton of Zoantharian corals is therefore purely derived 

 from an external exudation, and in this respect it contrasts most 

 markedly with the skeleton of Alcyonaria. In Ins paper on Eenilla 

 (1883), Wilson has described the origin of a typical Alcyonarian 



FIG. 68. Five stages in the development of tlie skeleton of Caryophyllia cyatlnts. 

 (After von Koch.) 



A, the six primary areas of deposit coalescing to form a star-sliaped figure. B, the thecal wall 

 just formed. C, the thecal wall thickened opposite the bases of the septa. D, the appearance of the 

 secondary septa and of the columella. E, tertiary septa formed, col, columella ; si, primary septum ; 

 s 2 , secondary septum ; s 3 , tertiary septum ; tit, wall of theca. 



skeleton. The lower cells of the ectoderm, corresponding roughly 

 in position to the interstitial cells of Hydra, acquire calcareous 

 concretions in their interior and migrate into the jelly. These 

 concretions form the characteristic Alcyonarian spicules. But in 

 Renilla, oval, wine-red, calcareous bodies are also formed in endodermal 

 cells, and both kinds of spicules are found in the adult. In their 

 characteristic skeleton therefore, as has already been said, lies the real 

 distinguishing mark of Alcyonaria. 



IV. CTENOrHORA 



The fourth group of Coelenterata differ profoundly from all the 

 rest, not only in their completely pehgic life, with no trace of a fixed 



