ANTHOZOA. ZOANTHARIA 77 



The Zoantharia comprise, (1) the sea-anemones, which 

 have usually been grouped together as the Actinaria, and 

 are unknown in the fossil state, since they possess no hard 

 parts; (2) the Antipatharia — colonial forms in which the 

 skeleton consists of an internal horny rod secreted by the 

 ectoderm ; these also are not found fossil ; (3) the Madre- 

 poraria, including the well-known stony corals, which are 

 very abundant as fossils. 



MADREPORARIA 



The polyp of a Madreporarian coral has essentially the 

 same structure as a common sea-anemone, but the ecto- 

 derm of the lower part of the body secretes a skeleton 

 consisting of carbonate of lime (fig. 22, 8, 9). The entire 

 skeleton is spoken of as the corallum, and in compound 

 corals the skeleton of each individual is termed a corallite. 

 The parts of the skeleton may be solid, or they may be 

 perforated, or formed of a network of rods. 



In a typical simple coral (fig. 25) the skeleton has a 



more or less conical form ; the 



base of the cone, on which the jt£\ 1 l/^< 



polyp is placed, is usually de- ^\\ ' /^a 



pressed, and is termed the calyx. L^ _nt--- — J-c 

 The wall bounding the corallum a \^^/ 1 Vvi 

 is known as the theca (fig. 22, 9; >£'/ \ I \^^~ 



fig. 24, d) ; sometimes there is, *> ' ^*4-L>^ 



outside this, another calcareous Fi g; 24 ; Diagrammatic sec- 

 tion (horizontal) 01 a simple 

 layer, the epitheca. The whole coral, a, columella ; b, pri- 



space enclosed by the theca is nTss^ments^ 115 f/ ' theCa; 

 termed the visceral chamber) it 



is divided up by various partitions, the most important 

 of which are the septa (fig. 22, 11 ; fig. 24, b). These are 



