HYDR^, JELLYFISHES, SEA ANEMONES, CORAL POLYPS 47 



iniier ends extended into narrow prolongations that run 

 at right angles to the body of the cells. These narrow pro- 

 cesses are called the muscle processes because they possess 

 highly developed contractile power, and they serve to con- 

 tract and expand the body. 



The body of the hydra is attached by means of a sticky 

 substance secreted by the posterior end, or foot. At times 

 the hydra breaks this connection and moves in a peculiar 

 manner to another location. It stretches out as far as 

 possible, bends over, and grasps hold with its tentacles. 

 Then it pulls the foot of the body close to the place where it 

 has taken hold, thus forming itself into a loop. By repeat- 

 ing this action, it moves from place to place after the man- 

 ner of a " looping," or " measuring" worm. At other times 

 it takes hold with the tentacles and swinging the posterior 

 end of the body slowly through the water turns a complete 

 somersault. Then, occasionally, it crawls slowly along by 

 means of its tentacles alone. 



Structure of the body. — The body of the hydra is a 

 long, cylindrical sack, closed at one end and open at the 

 other. The walls of the body are composed of three layers 

 of tissue: an outer layer of cells, the ectoderm; an inner 

 layer of cells, the endoderm; and between these two layers, 

 a third layer, of gelatinous, non-cellular tissue called the 

 mesogloea. The body cavity, or digestive cavity, since in 

 this animal they are one and the same, extends throughout 

 the whole length of the body and opens outward through 

 the mouth between the bases of the tentacles. The tenta- 

 cles are also hollow and open into the body cavity (Fig. 17). 



Method of obtaining food. — There are in the ectoderm of 

 the body, and especially of the tentacles, certain large cells, 

 known as the stinging thread cells. Each stinging cell con- 



