470 



EVOLUTION OF ANIMALS 



Part V 



Movements and Locomotion of Hydra. When they are searching for food 

 hydras sway their tentacles and stretch them gently in all directions. They 

 move from place to place, imperceptibly by gliding upon their bases, some- 

 times by turning somersaults (Fig. 23.5). Such end-over-end steps are re- 

 peated again and again. Green hydras move about more than other species; 



^ 





Fig. 23.3. A hydra which has caught 

 and swallowed a "full meal" of water flea. 

 Sketched from life. 



the brown and gray ones will attach themselves and sway or hang almost 

 motionless in one place for long periods. 



Responses and Coordination of Behavior. Hydras react to mechanical con- 

 tacts, light, electricity, and chemical solutions. The firmness of their attachment 

 to the side of an aquarium as they swing out in the water regardless of gravity 

 is an example of their reaction to contact. They respond to the slight current 

 created by a passing water flea with the simultaneous contraction of the ten- 

 tacles and the body, showing how quickly the reaction spreads through the 

 animal. In unevenly lighted jars, hydras will retreat from the dark areas as 

 well as from the strongly lighted ones moving about until by trial and error 

 they finally reach their optimum degree of light. All of these responses may be 

 affected by some special physiological state of the animal. 



Form and Structure. General Plan. The radial symmetry of hydra is at 

 once conspicuous in the arrangement of the tentacles (Fig. 23.3). It has a 

 distinct oral end with some of the characteristics of a head. The other end 

 functions as the base by which it is attached and on which it glides about. One 

 end of hydra is permanently different from the other, a foreshadowing of the 

 polarity so evident in higher animals. The front end of an ameba is distinguish- 

 able mainly by the fact that it is forward during locomotion. Hydra's bodily 



