i8 CORALS 



Coral, however, being generally used in a very indefinite 

 way, may mean in our common language either the dried 

 calcareous skeletal structure alone or the whole living 

 organism with hard skeleton and fleshy organs complete. 

 It is therefore necessary to use the term " Corallum " when 

 we desire to refer to the calcareous structures only, in contrast 

 to the soft flesh}' tissues that give rise to them. 



To return to our system of classification. 



The polyp-bearing corals belong to two widely separated 

 divisions {i.e. Phyla) of the Animal Kingdom, called the 

 Coelenterata and the Polyzoa. 



It is not necessary to relate in detail the many anatomical 

 and embryological differences between these two Phyla, for 

 which reference should be made to one of the many good 

 text-books of General Zoology. But there are two essential 

 points to which attention may be called. 



In the Coelenterata the mouth leads into a large un- 

 divided cavity in which the food is digested, and the in- 

 digestible parts of the food are ejected by the same aperture. 

 In the Polyzoa the mouth leads into a stomach and intestine, 

 and the indigestible parts of the food are ejected by an anal 

 aperture which is quite distinct from it (Fig. 74, p. 158). 

 There is thus a complete alimentary canal in the Polyzoa 

 which is without any direct communication with the body 

 cavity surrounding it. 



In all the Coelenterata without exception the tentacles 

 are provided with remarkable vesicles of microscopic size 

 called " Nematocysts," and these have the power of inflicting 

 a sting which kills or paralyses small animals that pass by 

 and captures them by means of a thread that is discharged 

 at the same time.^ The Coelenterata are therefore animals 

 that capture their prey by stinging them, and hence the 

 name Cnidaria (from KviS7] = Si nettle) is sometimes applied 

 to them. 



In a few corals {e.g. Millepora) the nematocysts are 

 powerful enough to penetrate the human skin, causing a 

 painful form of nettle-rash, but as a general rule living 

 corals can be handled freely without any ill effects. 

 1 See Fig. 69 on p. 14S. 



