CCELENTERATA ACTINOZOA. 



127 



181. The "reef-building corals" differ from the sea-ane- 

 mone mainly in having the power of secreting within their 

 tissues a skeleton composed of carbonate of lime. This 

 skeleton is termed the corallum. Tn its simplest form it 

 consists of a calcareous cup. of a cylindrical or conical 



FIG. 53. FUXGIA, a simple form of Coral, inhabited by a single Polype only. 



form, with radiating lamellae called septa. Some of these 

 septa reach and others fall short of a central column 

 which is generally present. The outer cup or theca (Gr. 

 theke, a sheath), is secreted by the inner surface of the 

 endoderm of the body wall ; the septa, by the radiating 

 partitions or mesenteries. The corallum is thus a cast 

 in carbonate of lime of the polype that forms it. 



The reef-building corals are generally compound animals, 

 forming colonies by budding and sell-division. New 

 colonies are formed by minute ova, which are produced 

 at certain seasons. Like the embryos of the sea-anemone, 

 they are able to swim about by the action of their cilia. 

 They are united by a common living substance, or 



