114 



COLLEGE ZOOLOGY 



in a new direction; hungry specimens are 

 more active than well-fed individuals. 



Locomotion 



When going from one place to another, 

 the hydra uses several methods. One is a 

 gliding movement, with the basal disk slowly 

 sliding over the object to which the animal 

 is attached. A second method is a "measur- 

 ing worm" type of movement (Fig. 55), in 

 which the hydra bends over and attaches its 

 tentacles to a region, slides its basal disk up 

 close to them, and then releases the tenta- 

 cles, assuming an upright position. Another 

 method is by turning somersaults. The ani- 

 mal releases its basal disk and moves it com- 

 pletely over and attaches it to a new region. 

 Such end-over-end movements are repeated 

 again and again. At other times the hydra 

 travels by a method seldom observed. It 

 moves from place to place in an upside down 

 position, using its tentacles as legs. To rise 

 to the surface of water, it may form a gas 

 bubble on its basal disk, which helps to 

 carry it upward. 



Contact 



Hydra reacts to various kinds of special 

 stimuli. Mechanical shocks, such as jarring 

 the watch glass containing the specimen, or 

 agitating the surface of the water, cause a 

 rapid contraction of a part of or the whole 

 animal. This is followed by a gradual ex- 

 pansion until the original condition is re- 

 gained. 



Mechanical stimuli may be localized or 

 nonlocalized. The one just mentioned is of 

 the latter type. Local stimulation may be 

 accomplished by touching the body or ten- 

 tacles with the end of a fine glass rod. It 

 has been noted that the stimulation of one 

 tentacle may cause the contraction of all the 

 tentacles, or even the contraction of both 

 tentacles and body. This shows that there 

 must be some sort of transmission of stimuli 

 from one tentacle to another and to the 

 body. The structure of the nervous system 

 would make this possible. 



Light 



There is no well-defined response to light, 

 although the final result is quite decisive. If 

 a dish containing hydras is placed so that 

 the illumination is not equal on all sides, 

 the animals will collect in the brightest 

 region. However, if the light is too strong, 

 they will congregate in a place where the 

 light is less intense. The hydra therefore 

 has an optimum with regard to light. The 

 movement into or out of a certain area is 

 accomplished by a method of trial and error. 

 When put in a dark place, the hydra be- 

 comes restless and moves about in no defi- 

 nite direction; but if white light is encoun- 

 tered, its locomotion becomes less rapid and 

 finally ceases altogether. The value of such 

 a reaction is considerable, since the small 

 animals that serve as food for it are attracted 

 to well-lighted areas. 



Other stimuli 



The reactions of the hydra to changes in 

 temperature are also indefinite, although in 

 many cases they enable the animal to escape 

 from a heated region. An attached hydra, 

 when subjected to a weak, constant electric 

 current, bends toward the anode, its body 

 finally becoming oriented with the basal 

 disk toward the cathode and the anterior 

 end toward the anode. The hydra does not 

 react to currents of water. 



The physiologic condition of an animal 

 determines to a large extent the kind of reac- 

 tions produced not only spontaneously, but 

 also by external stimuli. It determines 

 whether the hydra creeps upward to the 

 surface and toward the light, or sinks to the 

 bottom; how it reacts to chemicals and to 

 solid objects; whether it remains quiet in a 

 certain position, or reverses this position and 

 undertakes a laborious tour of exploration. 



Reproduction 



Reproduction takes place in the hydra 

 both asexually and sexually; in the former 

 case by budding, in the latter by production 



