68 ZOOLOGY 



The Hexactinia are divided into three groups : 



(i.) The AcTiNiAEiA, which are devoid of any kind of skeleton, 

 are usually single, and are mostly adherent to some 

 foreign tody ; occasionally they live lialf embedded in 

 mud or sand. Cerianthus, Actinia. 



(ii.) The Antipathaeia : they possess a horny axial skeleton 

 secreted from their ectoderm ; they are colonial, and form 

 large branching structures. Antipathes. 



(iii.) TJie Madeepoeaeia are solitary or colonial ; their most 

 remarkable characteristic is their power of secreting a 

 calcareous skeleton. The skeleton is often very massive, 

 and recent research has shown that it is entirely formed 

 from the ectoderm. Oculina, Astraea, Madrepora, 

 Fungia, Caryophyllia. 



The Madeepoeaeia are of the greatest importance in the 

 history of the earth. They are the true corals, and their 

 skeletons form hy far the greater part of the coral rock which 

 has huilt up a considerable portion of the earth's crust. Eeef- 

 forming corals do not as a rule grow below the forty-fathom 

 line, and are not usually found north or south of a belt ex- 

 tending 30° each side of the equator. 



As the coral grows, large masses of the coenenchyma or 

 common skeleton become covered over by the younger forma- 

 tions. This skeleton may be quite solid and dead, or it may 

 be pierced by canals which shelter coenosarcal tubes of living 

 matter, connecting one individual with another. This enables 

 us to divide the Madreporaria into two divisions : (i.) Perforata, 

 with the skeleton perforated by the cavities which lodge the coeno- 

 sarcal tubes, and (ii.) the Aporosa, in which no such perforations 

 exist. 



The form and shape of the skeleton is extremely varied, 

 and often complicated by the colonial habits of the actinozoan. 

 But whatever its shape, and however deeply it may have pene- 

 trated into the body of the soft gelatinous-looking animal, it is 

 always formed by ectoderm, and is consequently always outside 

 the animal, whose tissues are, as it were, moulded over it. 



