HYDRA AND CCELENTERATES IN GENERAL 143 



of corals are composed of calcium carbonate excreted by the ecto- 

 derm cells of the polyps. So numerous are these polyps that many 

 islands in the Pacific Ocean and many reefs near other islands con- 

 sist entirely of coral rock. The precious Red Coral is found only 

 in the Mediterranean Sea. 



Symbiosis. Symbiosis means an intimate and advantageous 

 association between two kinds of organisms. The most common 

 example is the lichen, which consists of two plants, an alga and a 

 fungus. Hydra viridis, the green Hydra, derives its color from a 

 great number of unicellular green plants, the Zoochlorella, which 

 occupy the basal portion of the entoderm cells. These green 

 algae manufacture starch in the presence of light, and, during 

 this process, liberate oxygen which is of advantage to the polyp. 

 Probably the Hydra also obtains food from these algae. The 

 security and carbon dioxide furnished by the protecting cells of 

 the polyp compensate the algae for the food and oxygen they 

 provide. 



A most complicated illustration of symbiosis is that of certain 

 hermit crabs with sea anemones. The hermit crab lives in the 

 shell of a large salt-water snail. As soon as a suitable shell is 

 found, the hermit crab takes possession. It then hunts about 

 until it finds a sea anemone, which it places upon the shell just 

 above the opening. Often the anemone completely covers the 

 hermit crab's house. The advantage to the sea anemone in this 

 partnership lies in the greater chances it has for proper food 

 conditions, since it is carried about from place to place by the 

 crab. The benefits to the latter are of a peculiar character. 

 Figure 68 shows a Ccelenterate colony, Hydractinia, con- 

 sisting of nutritive polyps with tentacles, reproductive individuals 

 bearing a circle of medusoid buds, spine-like protective members, 

 and bordering the edge of the shell, a row of threadlike defensive 

 polyps provided with stinging cells. When the hermit crab is 

 attacked, these stinging cells are shot into the enemy, which is 

 thus frequently driven away. In this way the Ccelenterate benefits 

 its associate. 



