82 



ANTHOZOA. ZOANTHARIA 



more or less separate, the lower parts are united. When 

 coenenchyma is present the polyps are united by an ex- 

 tension of the part which ordinarily occurs outside the 

 theca, and now forms a sheet called the coenosarc. 



Microscopic examination of thin sections shows that 

 each part of a coral is formed of thin layers or growth- 

 lamellae which consist of fine needle-like crystals placed 



Fig. 28. Transverse section of part of a septum and theea of Galaxea. 

 Highly magnified, d, dark spots ; g, growth- lamelhe; a, granule on 

 septum. (From M. M. Ogilvie.) 



more or less perpendicularly to the surfaces of the lamellae. 

 In a section of a dissepiment (fig. 27) a series of lines 

 parallel to the surface and other finer lines crossing at 

 right angles are seen — the former mark the growth- 

 lamellae, the latter the crystalline fibres. In a transverse 

 section of a septum (fig. 28) there is a median dark line 

 or row of dark spots, on each side of which the structure is 

 symmetrical. When the surface of the septum is plane, 



