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PHYSIOLOGY 



taches itself by flattening its surface against a rock until 

 the air is forced from beneath. The pressure of the air 



Fig. 80.— A, Coral showing living animal. CFrom Hertwig, after Heider.) B, Coral 

 skeleton. (From Hertwig, after Klunzinger.) C, Diagrammatic section of coral 

 showing flesh; above the line, ab, the section passes through the oesophagus, s ; 

 below the line it is lower down; the coral skeleton is black. (From Hertwig.) 



above then holds it to the rock. By muscular contrac- 

 tion it is able to creep slowly along. 



Fixed Forms. — Hydroids and corals are firmly at- 



