96 



ANTHOZOA. ALCYONARIA 



In some of the Alcyonaria, as for example Pennatula, 

 there are in addition to the ordinary polyps (or autozooids) 

 others of a more rudimentary character, known as siphono- 

 zooids, in which tentacles are absent. 



The blue coral (Heliopora) differs from other living 

 Alcyonaria in that the skeleton consists of calcareous fibres 

 instead of spicules, and in this respect resembles the 

 Madreporaria. Heliopora has the form of branched or 

 lobed masses, and is composed of tubes of two sizes ; the 



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Fig. 38. Tubipora musica, Recent. A, part of a colony, natural size. 

 B, diagrammatic vertical section of one corallite (enlarged) showing 

 canals in the wall and platform, a, corallite; b, platform; c, tabula. 



larger tubes or corallites are circular and possess usually 

 fifteen spine-like projections at their summits with ridges 

 below ; these are called pseudosepta, since they are not 

 related to the number of mesenteries and do not corre- 

 spond with true septa. The smaller tubes form a 

 coenenchyma between the corallites, and are more irregular 

 in form. Both corallites and coenenchymal tubes are 

 divided by horizontal plates or tabulae. The soft parts 

 form a thin sheet over the surface of the skeleton ; polyps 



