PHYLUM COELENTERATA (cNIDARIA). SIMPLE TISSUE ANIMALS 



125 



1IIIIIIIII Ectoderm 

 Iv/fS^^-x-:! Endoderm 

 Mesoglea 

 ■Tentacle 



Mouth 



Hydra (Hydroid polyp) 



Jellyfish (Medusa) 



Figure 63. Basic plans of the three chief forms of coelenterates. The mouths of the hydra 

 and sea anemone are held upward, but the jellyfish swims with mouth down. For purposes of 

 comparison, however, the jellyfish is drawn with its mouth up. 



ORIGIN AND RELATIONS 

 OF THE COELENTERATA 



The coelenterata probably arose from a 

 two-layered animal (Fig. 430). We can, of 

 course, only speculate regarding their origin 

 and differentiation. A hypothesis based on 

 our present knowledge is that coelenterates 

 developed from a free-swimming ciliated 

 form, something like the planula larvae of 

 certain hydroids (Fig. 57). This became 

 modified into a gastrula form with a body 

 wall consisting of an outer ectoderm, protec- 

 tive and sensor)' in function, and an inner 

 endoderm, digestive and absorptive in func- 

 tion. Between these layers a jellylike connec- 

 tive tissue, the mesoglea, appeared. The 

 gastrula ancestor possessed a central cavity, 

 the gastrocoel, a mouth, and a sense organ 

 opposite the mouth. The muscle cells and 

 nervous system were in a primitive stage of 

 differentiation. Tentacles grew out from 

 such an ancestral form, resembling some- 

 what a medusa. The larvae of these medusa- 

 like ancestors may have become attached 

 and then modified into hydroid polyps. Ac- 

 cording to the above hypothesis, the hydra is 

 not a primitive type, but a coelenterate, well 



developed histologically, that has lost its 

 medusa stage. 



RELATIONS OF THE 

 COELENTERATA TO MAN 



Coelenterates are of considerable eco- 

 nomic importance, though probably little 

 used as food by man. However, some scypho- 

 zoan coelenterates are eaten in the Orient, 

 and two species of Anthozoa arc eaten in 

 Italy under the name of Ogliolc. Precious 

 corals (Fig. 66), usually bright red or pink, 

 are made into necklaces and other tvpcs of 

 jewelry. 



Coral polyps build various types of reefs, 

 atolls, and islands. These are confined to 

 waters at least 60° F., principally in tropical 

 seas. The best-known coral islands are the 

 Maldive Islands of the Indian Ocean, Wake 

 Island, Marshall Islands, the Fiji Islands of 

 the Pacific Ocean, and those located in the 

 Bahama Islands region. Bermuda is a coral 

 island and the houses are built of coral 

 blocks mined from certain areas. 



The Mariana Islands are coral islands of 

 historic interest, for it was from an airfield 

 on one of them (Tinian Island) that the 



