MADKEPORARIAN CORALS 



27 



form, extend outwards radially, but do not reach the walls 

 of the theca. These are the " Pali." 



In the English cup-coral the septa are seen to project 

 as crests a short distance above the rim of the theca, and 

 are continued outside the rim as blunt ridges with granular 

 edges, called the " Costae " or " ribs " (Fig. 4). Below the 

 visible costae the outside of the theca is covered with a 

 chalky deposit which extends outwards over the spreading 

 ba-se of attachment, and this chalky deposit is called the 

 " Epitheca.''' 



The polyp (see Fig. i, p. i5) which forms this corallum 

 is in appearance very much like a sea-anemone. In a 



Fig. 3. — Diagram of a trans- 

 verse section through the cup of 

 a Caryophyllia. C, costa ; Co, 

 columella ; p, palus ; 5, septum ; 

 t, thecal wall. 



Fig. 4. — Diagram of an external view 

 of a Caryophyllia to show the theca {t} 

 with its projecting costae and the epi- 

 theca let). 



position corresponding with the centre of the cup there is 

 a slit-shaped mouth surrounded by a flat disc. At the 

 margin of the disc there are about fifty tentacles. Each 

 tentacle is provided with a number of wart-like batteries 

 of nematocysts and has a prominent white knobbed ex- 

 tremity, which is crowded with these stinging organs. 



When the polyp is dissected it is found that the mouth 

 leads into a short throat called the " Stomodaeum," and 

 this communicates with the general body cavity. The 

 stomodaeum is bound to the body wall by a number of 

 vertical fleshy bands called the " Mesenteries," and con- 

 sequently the appearance of a coral polyp in transverse 

 section has a resemblance to a cart-wheel, the stomodaeum 

 representing the hub and the mesenteries the spokes (Fig. 7, 



