146 TEXTBOOK OF ZOOLOGY 



The common tropisms, which have been described previously, are 

 present in hydras. They respond to light and will find an optimum 

 intensity which varies with the different species. Green hydras 

 react positively to sunlight and withstand moderate temperature; 

 hence they are adapted to the Southwest. They likewise possess 

 an optimum for temperature and prefer relatively cool water. They 

 seem not to become particularly uncomfortable until the tempera- 

 ture gets up to 31° C. ; then they attempt to find lower temperature. 

 As the temperature is lowered on them, they simply become less 

 and less active and finally cease to move as the freezing point is 

 approached. As pointed out previously, both chemotropisms and 

 thigmotropism are concerned in food-taking. Contact stimuli are 

 of considerable significance in a sedentary animal like this. It re- 

 mains attached in contact with some solid body most of the time. 

 Sudden mechanical stimulation like stirring the water or jarring 

 the attachment of the animal will cause it to contract vigorously. 



Locoynotion is accomplished in at least four ways. Gliding from 

 one point to another by partially releasing the basal disc and slip- 

 ping it to a new location is common. Or the animal may bend over 

 and cling to the substratum by the tentacles, release the basal disc, 

 then draw the body toward this point, where the basal disc is reat- 

 tached. This process is consecutively repeated and is called "loop- 

 ing." Occasionally the animal bends over, holds by the tentacles, 

 then turns a "handspring" or "somersault" to attach the basal disc 

 on the substratum beyond this point. The fourth means by which 

 locomotion is effected is by dropping to the bottom, then secreting a 

 bubble of gas at the basal disc and floating back to the top on that. 



External Anatomy 



Hydra is a macroscopic animal, but it is relatively small. Its body 

 is quite contractile, being able to extend from a contracted length 

 of two or three millimeters to a length of eighteen or twenty milli- 

 meters. The column or body is a tubular, cylindrical trunk which 

 ordinarily stands in a vertical position. In some forms the distal 

 (free, oral, or anterior) end of the column is much stouter than 

 the proximal (attached, aboral, or posterior) end, but in H. viridis- 

 sima there is only a slight tapering toward the basal end. Attached 

 around the free end of the column is a circlet of from four to seven 

 fingerlike tentacles, which extends free in the water. Tentacles may 



