COELENTERATA 69 



Cambrian to the present. Corals are composed of CaCOa and so 

 are well preserved. Sea anemones and Ctenophores are not 

 preserved as fossils. 



Ancestry and Relationship to Other Phyla. — The lowest Coe- 

 lenterate form known is the simple hydrozoan polyp, represented by 

 Hydra and by the hydrula stage of many Hydrozoa. Scyphozoan 

 polyps are represented by the scyphula of Aurelia, which is more 

 complex because of the stomodaeum, gastric ridges and filaments. 

 The Actinozoan polyp or actinula is more complex still. 



The hydroids have adopted asexual multiplication by budding 

 during the larval stage. Certain of the zooids become medusae^ the 

 rest retaining the polyp form and furnishing nourishment for the 

 asexual colony. 



The relationships of the Ctenophora to other Coelenterata are 

 doubtful. While the absence of stinging capsules and the presence 

 of collared endoderm cells in the Porifera places them in a separate 

 Phylum, it is assumed that they were derived from the Protozoan 

 ancestors of the Coelenterates. 



Economic Importance of Coelenterates 



Hydrozoa. — Hydra is an enemy of mosquito and other insect 

 larvae, but of relatively small importance in such a role. It also 

 attacks trout fry. Hydra is an enemy of annelids {Tubifex) and 

 small Crustacea such as Daphnia, which are important food for 

 fishes. The hydroids are food for fishes. Polypodium is in early life 

 parasitic on the eggs of the sturgeon. Scyphozoa are eaten in Japan 

 and the Philippines, preserved in salt or between oak leaves. 

 Sertularia are sometimes sold as " air-plants." 



Actinozoa. — Sea anemones have been used as food by the Italians 

 for many years. They are sold under the name of " ogliole." 

 When fried in oil they are said to be very palatable. In the West 

 Indies, a coral-like form called the " sea-ginger " is esteemed as a 

 condiment. 



Corals are the only Coelenterates of great importance economi- 

 cally. Coral reefs are formed by the limestone secretions of in- 

 numerable animals resembling anemones somewhat in structure. 

 Sometimes reefs of coral surround islands which submerge (Darwin's 

 theory, page G^) and leave a " lagoon " that proves a safe haven 

 for ships. But, in many cases, reefs are dangerous liabilities in 



