CCELENTERATES 



201 



of the cavity. Part of the cavity, as in the hydra, is given up to digesting 

 the food. Food, which is often taken ahve into the body, is killed by means 

 of stinging cells found in the long 

 threadlike tentacles developed 

 near the base of the cavity. 



The partitions or mesenteries, 

 as they are called, are usually 

 double. These perform in the 

 coral polyp, which is like a mini- 

 ature sea anemone in structure, 

 a very important function. 



Coral} — (Madreporic coral or 

 our common Astrangia may be 

 used for observations.) Test with 

 a drop of hydrochloric acid. 

 What substance is present? Ex- 

 amine a piece of the ordinary 

 white branching coral (madre- 

 pore) with the hand lens. Notice 

 the little holes at intervals over 



the surface 

 holes ? 



A branching madreporic coral. 



Do you find little partitions in these 



These cuplike depressions were once occupied 

 by the coral animals or polyps, each in its own 

 cup. The mesenteries of the coral polyp are double 

 and hollow on the under surface. The partitions 

 seen in the coral cups lie between these mesenter- 

 ies, and are formed by them when the animal is alive. 

 Sea water has a considerable amount of lime in its 

 composition. This lime (calcium carbonate) is 

 taken from the water by certain of the cells of the 

 coral polyp and deposited around the base of the ani- 

 mal and between the mesenteries, thus giving the 

 appearance just seen in the cups of the coral branch. 

 Asexual Reproduction. — These polyps re- 

 produce by budding, and when alive cover the 

 whole coral branch with a continuous living mass of polyps, each connected 

 with its neighbor. In this way great masses of coral are formed. Coral, 

 in a living state, is alive onlj?" on the surface, the polyps building outward 

 on the skeleton formed by their predecessors. 



Economic Importance of Corals. — Only one (Astrangia) of a great 

 many different species of coral lives as far north as New York. In tropical 

 waters they are very abundant. Coral building has had and still has an im- 

 mense influence on the formation of islands, and even parts of continents in 



A single coral cup, showing 

 the walls of lime built by 

 the mesenteries. From 

 a photograph loaned by 

 the American Museum 

 of Natural History. 



1 See Hunter and Valentine, Manual, page 157. 



