142 THE GREAT BARRIER REEF. 



belonging to this group, or order, lies in the fact that the prehensile organs, or tentacles, 

 which surround the mouth of each individual polyp are always eight in number, and have 

 their edges fringed with secondary lobes or filaments ; in rarer instances, they are distinctly 

 warted. In all ordinary sea-anemones and the more typical coral polyps the tentacles are 

 a multiple of six. Very frequently they are only twelve, but more commonly from twice to 

 five or six times that number. In the small group of the Antipatharia, as hereafter shown, 

 the tentacles are usually as few as six. The individual tentacles, moreover, of all the coral- 

 producing species, and of the majority of the skeletonless anemones as well, are perfectly 

 simple. There are, however, exceptional forms of sea-anemones, or Actinias, in which the 

 prehensile tentacles are of even more complex structure than those of the eight-rayed 

 Alcyonaria. 



The compound nature of the coral organisms which are most extensively associated with reef- 

 construction has been already explained. Each of these compound growths, however, began 

 its existence as a single individual polyp, and it is by a continual process of what is known as 

 gemmation or budding — the new animal buds remaining in close union with the parent polyp, 

 and, again, giving birth to successive buds — that the characteristic compound corallum is ultimately 

 formed. The Mushroom-corals, genus Fitngia, illustrate the more exceptional instances in 

 which the coral polyps remain single in their fully matured condition. There are, on the other 

 hand, certain exceptional forms of the skeletonless sea-anemones, or Actinia, which, through the 

 continued adhesion of the buds, form more or less extensive composite communities. These 

 exceptional types, hereafter enumerated, suffice to show that both the soft bodied anemones 

 and the coral-forming series, embrace simple or individually distinct forms, in which the connection 

 between parent and offspring is early lost, and compound or colonial ones, in which the connection 

 is retained. 



For ■ the convenience of study and S3'stematic classification, the entire series of polyp- 

 animals or Coelenterata, is subdivided into two primary classes and several subordinate groups 

 .or orders, whose most distinctive features are indicated in the following table. — 



SUB-KINGDOM CCELEETERITS, OR POLYP-IN I Mi? LS. 



CLASS I.— -HYDROZOA, OR HYDROID POLYPS. 



Including all Hydroid Zoophytes, most jelly-fishes, and a few coral-constructing species, 



MiUeporidas. 



CLASS II.— ACTINOZOA. 



Including all sea-anemones and true coral-constructing polyps; separable as a whole into 



the several following orders : — 



ORDER I.— ACTINARIA. 

 Including all ordinary, isolated, sea-anemones. 



