76 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



vertical plane and unite to form a beautiful network 



(Fig. 35). 



Coral. The majority of Actinozoa secrete a calca- 

 reous or horny framework called " coral." With few 

 exceptions, they are fixed and composite, living in colo- 

 nies formed by a continuous 

 process of budding. Their struc- 

 tures take a variety of shapes ; 

 often domelike, but often resem- 

 bling shrubbery and clusters of 

 leaves. The members of a coral 

 community are organically con- 

 nected; each feeds himself, yet 

 is not independent of the rest. 

 The compound mass is " like a 

 living sheet of animal matter, 

 fed and nourished by numerous 

 mouths and as many stomachs." 

 - Life and death go on together, 

 the old polyps dying below as 

 new ones are developed above. The living part of an 

 Astrcea is only half an inch thick. The growth of the 

 branching Madrepora is about three inches a year. The 

 colors of the coral polyps are brilliant and varied, being 

 green, purple, pink, or brown. The organ-pipe coral has 

 green polyps and crimson skeleton, while the precious 

 ,sx2\(Corallium) has white polyps and a bright red axis. 

 Another kind is bright blue. The usual size varies 

 from that of a pin's head to half an inch, but the 

 mushroom coral (which is a single individual) may be 

 a foot in diameter. 



Corals are of two kinds : those deposited within the 

 tissues of the animal (sclerodermic\ and those secreted 

 by the outer surface at the foot of the polyp (sclero- 

 basic). The polyps producing the former are actinoid, 



FIG. 27. Organ-pipe coral (Tu 



pora ntusica). Indian Ocean. 



