390 The Animal Kingdom 



feeding, high temperatures, or other causes. At these times, there is 

 a lowering of the metabolic rate, and the resulting condition is known 

 as depression. Following this, there is a breakdown and reabsorption 

 of tissues. Recovery from this depression may occur spontaneously. 



The hydra is able to move about in its trial and error search for 

 food and favorable light and temperature conditions. The movements 

 are accomplished in a variety of ways. One common method is that 

 of sliding along on the basal disc. More rapid movements may be 

 accomplished by having the tentacles loop over and attach by means 

 of the glutinant nematocysts. The basal disc is released and the animal 

 stands on its "head" momentarily while the disc loops to a new position. 

 It may continue this looping movement for a considerable distance. At 

 times, the tentacles act as legs to the inverted animal. Hydra also can 

 raise and lower itself in the water. In rising, it usually imprisons a 

 small bubble of gas which is secreted by the cells of the basal disc. When 

 lowering, the tentacles may fold up for a rapid descent or spread out for 

 a slower one. Often the hydra attaches itself to the undersurface of the 

 surface water film, and moves about on this thin "skin." 



The hydra is carnivorous in its food habits, and feeds chiefly on 

 animals smaller than itself, such as the microcrustacea which are cap- 

 tured by means of the nematocysts. When hydras become too thick in 

 an area they may actually be able to kill small fish. 



Regeneration. — The ability of hydras to regenerate v/as first 

 demonstrated by an English scientist, Trembley, in 1744, and has since 

 been demonstrated countless times by curious scientists and eager 

 students. First Trembley noted that the animal could be cut in two in 

 any direction, and shortly each half would grow into a perfect hydra. 

 If a longitudinal cut were not complete, the two halves would simply 

 fuse as if nothing had happened. In fact, by a clever bit of manipulation, 

 he actually turned the hydras inside out and found that they survived. 

 It has since been shown that when the hydra is turned inside out, all 

 the epidermal cells start moving outward, and the gastrodermal cells 

 start moving inward. They pass by each other, and soon the hydra is 

 its old self once again. If small pieces are separated, then allowed 

 to come together again, they will fuse ; however, it has been shown that 

 gastrodermis can fuse only with other pieces of gastrodermis and 

 epidermis only with epidermis. 



Two-headed hydras can be produced by splitting one longitudinally 

 nearly to the base and preventing the two parts from fusing. Each side 

 will develop an oral end. 



