ANTHOZOA 



241 



tube by a secretion of the column, still others secrete calcare- 

 ous matter, especially from the surface of the basal disc. This 

 is notably the case with the corals. Through this secretion 

 the animal becomes immovably fixed. The basal disc of the 



Fig. 137. — A branching madrepore coral, Astrangia. Slightly reduced. 



coral polyp secretes more rapidly along its edge so thai a cup 

 is formed into which the animal can more or less r()nij)lelely 

 withdraw. Within this cup there are also vertical phites and 

 pillars built up by the unequal secretion of the various parts 

 of the base. To this is due the beautiful structure of many 

 16 



