CORALS. 



507 



in which the coral-stocks grow, we can only select a few types. In Dendropliyllia, 

 as shown in the illustration, we have a tree-like growth ; each polyp secreting a 



solid pedestal for itself, 

 and living in a depres- 

 sion in the top. This 

 is shown in the section 

 B. Into this depres- 

 sion the soft animal 



A BRANCHING CORAL, Dendrophyllia. 

 A, Terminal branch of a stock (nat. size) ; B, Longitudinal section of a single polyp (magnified). 



can withdraw at the approach of danger, drawing all its tentacles (which also 

 contract) down to a. The space occupied by the animal is not very roomy at 

 the best, and it is further limited both by a great columella d, rising up in its 



interior, and by the solid septa b projecting into it 

 all round. It must not be forgotten that these parts 

 are not in the animal but outside of it, and as they 

 are secreted they push the skin up and never pene- 

 trate into the tissues themselves. These polyps bud 

 at intervals, the apical polyp most frequently ; and 

 the result is a simple branched stock, as seen. 



A different kind of stock is developed when the 

 polyps produce many buds, as in the madrepores. 

 In these delicate stocks, selected polyps spring up 

 above the rest, and their sides become covered with 



small buds. Space 

 would obviously 

 not permit all 

 these small buds 



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the same manner. 

 A few favoured 

 ones, however, 

 which have suffi- 

 cient room next 

 spring out and 



A MADREPORE CORAL (Modrqpora vemtcosa). COV eret * 



In the section B the canals which connect the polyps through the stony skeleton are seen, again With small 



