68 ZOOLOGY 
The Hexactinia are divided into three groups : 
G1.) Zhe ACTINIARIA, which are devoid of any kind of skeleton, 
are usually single, and are mostly adherent to some 
foreign body ; occasionally they live half embedded in 
mud or sand. Cerianthus, Actinia. 
(ii.) Zhe ANTIPATHARIA : they possess a horny axial skeleton 
secreted from their ectoderm ; they are colonial, and form 
large branching structures. Antipathes. 
(iu.) Zhe MADREPORARIA are solitary or colonial ; their most 
remarkable characteristic 1s their power of secreting a 
calcareous skeleton. The skeleton is often very massive, 
and recent research has shown that it is entirely formed 
Jrom the ectoderm. Oculina, <Astraea, Madrepora, 
Fungia, Caryophyllia. 
The MADREPORARIA are of the greatest importance in the 
history of the earth. They are the true corals, and their 
skeletons form by far the greater part of the coral rock which 
has built up a considerable portion of the earth’s crust. eef- 
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: (1.) 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. 
