30:2 ELEMENTS OF COMPARATIVE ZOOLOGY. 



whole colony. In some the outer surface of the body is 

 naked, but in many of the solitary and in most of the colo- 

 nial forms the base or both base and column secrete car- 

 bonate of lime, thus forming a solid support for the body. 

 This solid support is the well-known coral. In most spec- 



FIG. 135. -Diagram of a bit of coral to show the FIG. 136. Section of a 

 way in which the polyps are connected. The coral cup showing the 

 coral is black ; the digestive cavity shaded. calcareous septa. Af- 



ter Pourtales 



imens of coral one can readily recognize the cups in which 

 the separate polyps were situated; and in these cups, in 

 most cases, are calcareous partitions much like the septa of 

 the soft parts.* As long as the colony remains alive it is 

 constantly budding off new polyps, and thus the colony and 

 the coral grow. Those species which live in cold water 

 produce but little coral, but in tropical waters the coral- 

 producing forms abound, and by their combined secretions 

 the coral islands are made. 



The great majority of the Actinozoa may be subdivided, 

 according to the number of septa, into two orders: 



* These calcareous septa do not coincide with, but alternate in posi- 

 tion with, the fleshy septa. 



