PHYLUM COELENTERATA 143 



Astrangia is the common coral polyp, and it is quite similar to a 

 small sea anemone to which calcium carbonate has been added by 

 secretion from the ectoderm cells as well as having budded to form 

 a colony of numerous individuals. Coral poljqDS vary in size from 

 one-sixteenth of an inch to several inches in diameter. In time 

 continually growing colonies of these animals can produce enormous 

 stony barriers (reefs) in the sea. One such reef is over 1,100 miles 

 in length and from ten to twenty-five fathoms deep. Many corals 

 are of beautiful colors. 



Order Antipathidea. — An order composed of branching colonies 

 whose individuals are joined by a branched tubular axis which is 

 covered by an epidermal layer. Cirripathes and Antipathes are typi- 

 cal examples. 



srri,.^^.^ 



•Trv^.- ^»* 





A 



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Fig. 67. — Common coral, Astrangia danae. A, Stone produced by the animals 

 when cleaned ; B, polyps in natural habitat. 



Subclass Alcyonaria. — The features of this division include eight 

 hollow, feathered tentacles, eight mesenteries, and one siphonoglyphe. 

 Colonial and pol^'morphic forms are not uncommon. 



Order Alcyonacea. — A colonial group which has calcareous spicules 

 but lacks an axial rod. Body walls of individuals fuse together as 

 one. Alcyoninm is the type example. Organ pipe coral belongs in 

 this order. 



Order Oorgonacea. — This is another colonial coral which is sessile 

 and has a calcareous axial rod. The colonies are bilaterally sym- 

 metrical. The common sea fan, Gorgonia, as well as the precious 

 Corallium rubrum are well known examples. 



Order Pennatidacea. — Another colonial form whose body is modi- 

 fied so that one portion is submerged in the substratum. The colony 



